Table binding does not work with `dataType` set to `string`

From https://github.com/Azure/azure-functions-python-worker/pull/409:

*function.json*:

```json
{
"scriptFile": "__init__.py",
"disabled": false,

"bindings": [
{
"type": "httpTrigger",
"direction": "in",
"name": "req"
},
{
"direction": "in",
"type": "table",
"name": "docs",
"dataType": "string",
"partitionKey": "test",
"rowKey": "foo",
"tableName": "test",
"connection": "AzureWebJobsStorage"
},
{
"type": "http",
"direction": "out",
"name": "$return"
}
]
}
```

Exception:

```
fail: Function.table_input[0]
Executed 'Functions.table_input' (Failed, Id=7ad33ddf-12de-4d7b-8e54-cdc444fdfc12)
Microsoft.Azure.WebJobs.Host.FunctionInvocationException: Exception while executing function: Functions.table_input ---> System.InvalidOperationException: Table entity types must provide a default constructor.
at Microsoft.Azure.WebJobs.Host.Tables.TableClient.VerifyDefaultConstructor(Type entityType) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Extensions.Storage\Tables\TableClient.cs:line 75
at Microsoft.Azure.WebJobs.Host.Tables.PocoEntityArgumentBindingProvider.TryCreate(ParameterInfo parameter) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Extensions.Storage\Tables\PocoEntityArgumentBindingProvider.cs:line 24
at Microsoft.Azure.WebJobs.Host.Tables.CompositeEntityArgumentBindingProvider.TryCreate(ParameterInfo parameter) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Extensions.Storage\Tables\CompositeEntityArgumentBindingProvider.cs:line 23
at Microsoft.Azure.WebJobs.Host.Tables.TableAttributeBindingProvider.TryCreate(BindingProviderContext context) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Extensions.Storage\Tables\TableAttributeBindingProvider.cs:line 69
at Microsoft.Azure.WebJobs.Host.Tables.TableAttributeBindingProvider.<>c__DisplayClass5_0.b__0() in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Extensions.Storage\Tables\TableAttributeBindingProvider.cs:line 84
at System.Threading.Tasks.Task`1.InnerInvoke()
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location where exception was thrown ---
at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot)
--- End of stack trace from previous location where exception was thrown ---
at Microsoft.Azure.WebJobs.Host.Bindings.GenericCompositeBindingProvider`1.TryCreateAsync(BindingProviderContext context) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Bindings\BindingProviders\GenericCompositeBindingProvider.cs:line 61
at Microsoft.Azure.WebJobs.Host.Bindings.CompositeBindingProvider.TryCreateAsync(BindingProviderContext context) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Bindings\BindingProviders\CompositeBindingProvider.cs:line 25
at Microsoft.Azure.WebJobs.Binder.BindAsync[TValue](Attribute[] attributes, CancellationToken cancellationToken) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Bindings\Runtime\Binder.cs:line 97
at Microsoft.Azure.WebJobs.Script.Binding.FunctionBinding.BindStringAsync(BindingContext context) in C:\azure-webjobs-sdk-script\src\WebJobs.Script\Binding\FunctionBinding.cs:line 220
at Microsoft.Azure.WebJobs.Script.Binding.ExtensionBinding.BindAsync(BindingContext context) in C:\azure-webjobs-sdk-script\src\WebJobs.Script\Binding\ExtensionBinding.cs:line 75
at Microsoft.Azure.WebJobs.Script.Description.WorkerLanguageInvoker.<>c__DisplayClass7_0.<b__1>d.MoveNext() in C:\azure-webjobs-sdk-script\src\WebJobs.Script\Description\Rpc\WorkerLanguageInvoker.cs:line 95
--- End of stack trace from previous location where exception was thrown ---
at Microsoft.Azure.WebJobs.Script.Description.WorkerLanguageInvoker.BindInputsAsync(Binder binder) in C:\azure-webjobs-sdk-script\src\WebJobs.Script\Description\Rpc\WorkerLanguageInvoker.cs:line 98
at Microsoft.Azure.WebJobs.Script.Description.WorkerLanguageInvoker.InvokeCore(Object[] parameters, FunctionInvocationContext context) in C:\azure-webjobs-sdk-script\src\WebJobs.Script\Description\Rpc\WorkerLanguageInvoker.cs:line 56
at Microsoft.Azure.WebJobs.Script.Description.FunctionInvokerBase.Invoke(Object[] parameters) in C:\azure-webjobs-sdk-script\src\WebJobs.Script\Description\FunctionInvokerBase.cs:line 84
at Microsoft.Azure.WebJobs.Script.Description.FunctionGenerator.Coerce[T](Task`1 src) in C:\azure-webjobs-sdk-script\src\WebJobs.Script\Description\FunctionGenerator.cs:line 225
at Microsoft.Azure.WebJobs.Host.Executors.FunctionInvoker`2.InvokeAsync(Object instance, Object[] arguments) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionInvoker.cs:line 63
at Microsoft.Azure.WebJobs.Host.Executors.FunctionExecutor.InvokeAsync(IFunctionInvoker invoker, ParameterHelper parameterHelper, CancellationTokenSource timeoutTokenSource, CancellationTokenSource functionCancellationTokenSource, Boolean throwOnTimeout, TimeSpan timerInterval, IFunctionInstance instance) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionExecutor.cs:line 556
at Microsoft.Azure.WebJobs.Host.Executors.FunctionExecutor.ExecuteWithWatchersAsync(IFunctionInstance instance, ParameterHelper parameterHelper, ILogger logger, CancellationTokenSource functionCancellationTokenSource) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionExecutor.cs:line 503
at Microsoft.Azure.WebJobs.Host.Executors.FunctionExecutor.ExecuteWithLoggingAsync(IFunctionInstance instance, ParameterHelper parameterHelper, IFunctionOutputDefinition outputDefinition, ILogger logger, CancellationTokenSource functionCancellationTokenSource) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionExecutor.cs:line 439
at Microsoft.Azure.WebJobs.Host.Executors.FunctionExecutor.ExecuteWithLoggingAsync(IFunctionInstance instance, FunctionStartedMessage message, FunctionInstanceLogEntry instanceLogEntry, ParameterHelper parameterHelper, ILogger logger, CancellationToken cancellationToken) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionExecutor.cs:line 249
--- End of inner exception stack trace ---
at Microsoft.Azure.WebJobs.Host.Executors.FunctionExecutor.ExecuteWithLoggingAsync(IFunctionInstance instance, FunctionStartedMessage message, FunctionInstanceLogEntry instanceLogEntry, ParameterHelper parameterHelper, ILogger logger, CancellationToken cancellationToken) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionExecutor.cs:line 293
at Microsoft.Azure.WebJobs.Host.Executors.FunctionExecutor.TryExecuteAsync(IFunctionInstance functionInstance, CancellationToken cancellationToken) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionExecutor.cs:line 89
```