Unable to use "guid" as an HttpTrigger route parameter (Invalid cast from 'System.String' to 'System.Guid')

When trying to pass a guid as a parameter binding within a route definition the following exception is thrown

`Exception while executing function: TestParams -> One or more errors occurred. -> Exception binding parameter 'req' -> Invalid cast from 'System.String' to 'System.Guid'.`

#### Investigative information

Please provide the following:

- Timestamp: 2018-06-07T11:27:47.578
- Function App version (1.0 or 2.0-beta): 1.0.12.1
- Function App name: vos6functiondev
- Function name(s) (as appropriate): GetPendingDamages
- Invocation ID: (requestid): 65ead198-8c37-438d-b9a5-ac17165f1e41
- Region: UK West

#### Repro steps

Provide the steps required to reproduce the problem:

- For an HttpTrigger, configure a route parameter binding for a Guid

```
[FunctionName("GetPendingDamages")]
public static async Task Run(
[HttpTrigger(AuthorizationLevel.Function, "get", Route = "GetPendingDamages/{stayId:guid}")]
HttpRequestMessage req,
Guid stayId,
TraceWriter log)
{...}
```
- Invoke the fuction as a GET request
- The following exception will be thrown
` "message": "Exception while executing function: TestParams -> One or more errors occurred. -> Exception binding parameter 'req' -> Invalid cast from 'System.String' to 'System.Guid'.",`

#### Expected behavior

Provide a description of the expected behavior.

I would expect to either
- have a valid Guid value available as an input parameter
- receive a 400 Bad Request, in case the value could not be parsed as a Guid

#### Known workarounds

Provide a description of any known workarounds.

You can do a `Guid.TryParse()` in code, but it would be nice to be able to have the Guid bindings available like in Web API 2

as it has been reported here
http://dontcodetired.com/blog/post/Dynamic-Binding-in-Azure-Functions-with-Imperative-Runtime-Bindings
and here https://stackoverflow.com/questions/46743180/is-attribute-routing-possible-in-azure-functions

Thanks :)