Don't try to read HTTP request body as string if explicitly known as binary

Ideally, we would only try to read the body as JSON if its header was application/json.

As a non-breaking change though, we can check if the content [IsMediaTypeOctetOrMultipart](https://github.com/Azure/azure-functions-host/blob/dev/src/WebJobs.Script/Extensions/HttpRequestExtensions.cs#L101) and not [try to apply binding data to a json payload](https://github.com/Azure/azure-webjobs-sdk-extensions/blob/dev/src/WebJobs.Extensions.Http/HttpTriggerAttributeBindingProvider.cs#L293) if we find that that is true

Start with IsMediaTypeOctetOrMultipart in src/WebJobs.Script/Extensions/HttpRequestExtensions.cs and the binding-data path in WebJobs.Extensions.Http/HttpTriggerAttributeBindingProvider.cs. Check how binary or multipart content is handled before string or JSON processing, and verify that binding data is not applied to an explicitly binary payload.

Written by the indexing model from the issue text.