Unable to use simple configuration of HttpRequest with content-type application/x-www-form-urlencoded

When trying to use the HttpRequest Activity with content-type application/x-www-form, I've encountered the following issue :

When using a string variable with something like `key1=value1&key2=value2`

```
The JSON value could not be converted to System.Collections.Generic.Dictionary`2[System.String,System.String]. Path: $ | LineNumber: 0 | BytePositionInLine: 425.

at System.Text.Json.ThrowHelper.ThrowJsonException_DeserializeUnableToConvertValue(Type propertyType) at
System.Text.Json.Serialization.JsonDictionaryConverter`3.OnTryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, TDictionary& value) at
System.Text.Json.Serialization.JsonConverter`1.TryRead(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options, ReadStack& state, T& value, Boolean& isPopulatedValue) at
System.Text.Json.Serialization.JsonConverter`1.ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state) at
System.Text.Json.JsonSerializer.ReadFromSpan[TValue](ReadOnlySpan`1 json, JsonTypeInfo`1 jsonTypeInfo) at
System.Text.Json.JsonSerializer.Deserialize[TValue](String json, JsonSerializerOptions options) at
Elsa.Http.ContentWriters.FormUrlEncodedHttpContentFactory.GetContentAsDictionary(Object content) at
Elsa.Http.ContentWriters.FormUrlEncodedHttpContentFactory.CreateHttpContent(Object content, String contentType) at
Elsa.Http.SendHttpRequestBase.PrepareRequest(ActivityExecutionContext context) at
Elsa.Http.SendHttpRequestBase.TrySendAsync(ActivityExecutionContext context) at
Elsa.Http.SendHttpRequestBase.ExecuteAsync(ActivityExecutionContext context) at
Elsa.Workflows.Activity.Elsa.Workflows.Contracts.IActivity.ExecuteAsync(ActivityExecutionContext context) at
Elsa.Workflows.Middleware.Activities.DefaultActivityInvokerMiddleware.ExecuteActivityAsync(ActivityExecutionContext context) at
Elsa.Workflows.Runtime.Middleware.Activities.BackgroundActivityInvokerMiddleware.ExecuteActivityAsync(ActivityExecutionContext context) at
Elsa.Workflows.Middleware.Activities.DefaultActivityInvokerMiddleware.InvokeAsync(ActivityExecutionContext context) at
Elsa.Workflows.Middleware.Activities.NotificationPublishingMiddleware.InvokeAsync(ActivityExecutionContext context) at
Elsa.Workflows.Middleware.Activities.ExecutionLogMiddleware.InvokeAsync(ActivityExecutionContext context)
```

If we use a Json Variable and returning this variable (tested using Javascript Code) :
```
Object must implement IConvertible.

at System.Convert.ChangeType(Object value, Type conversionType, IFormatProvider provider) at
Elsa.Http.ContentWriters.FormUrlEncodedHttpContentFactory.GetContentAsDictionary(Object content) at
Elsa.Http.ContentWriters.FormUrlEncodedHttpContentFactory.CreateHttpContent(Object content, String contentType) at
Elsa.Http.SendHttpRequestBase.PrepareRequest(ActivityExecutionContext context) at
Elsa.Http.SendHttpRequestBase.TrySendAsync(ActivityExecutionContext context) at
Elsa.Http.SendHttpRequestBase.ExecuteAsync(ActivityExecutionContext context) at
Elsa.Workflows.Activity.Elsa.Workflows.Contracts.IActivity.ExecuteAsync(ActivityExecutionContext context) at
Elsa.Workflows.Middleware.Activities.DefaultActivityInvokerMiddleware.ExecuteActivityAsync(ActivityExecutionContext context) at
Elsa.Workflows.Runtime.Middleware.Activities.BackgroundActivityInvokerMiddleware.ExecuteActivityAsync(ActivityExecutionContext context) at
Elsa.Workflows.Middleware.Activities.DefaultActivityInvokerMiddleware.InvokeAsync(ActivityExecutionContext context) at
Elsa.Workflows.Middleware.Activities.NotificationPublishingMiddleware.InvokeAsync(ActivityExecutionContext context) at
Elsa.Workflows.Middleware.Activities.ExecutionLogMiddleware.InvokeAsync(ActivityExecutionContext context)
```

The `FormUrlEncodedHttpContentFactory` seems to have an issue when interpreting the input.

To be userfriendly, the input should be given using :
- key value pair in string like key1=value1&key2=value2
- could be a simple json object like `{"key1"="value1", "key2"="value2" }`

Start with Elsa.Http.ContentWriters.FormUrlEncodedHttpContentFactory, using the two stack traces to inspect GetContentAsDictionary and CreateHttpContent. Reproduce the issue through the HttpRequest Activity with a key-value string and a JSON object. Done means both supported input forms create form-urlencoded content without the reported conversion exceptions.

Written by the indexing model from the issue text.