When validation of MediaType fails, show all accept MediaType by the OpenApi document

[rodrigoramos]

Validation of Media Type (Content-type) fails when a single letter is different from OpenApi document and the Request data.

In order to understand better what's going on, would be better to add to the error message which media types are allowed by the Open Api document.

The piece of code which has to change is:
src/OpenApiContract.Validator/RequestValidator.cs:170

if (!requestBodySpec.Content.TryGetValue(content.Headers.ContentType.MediaType, out OpenApiMediaType mediaTypeSpec))
   throw new RequestDoesNotMatchSpecException($"Content media type '{content.Headers.ContentType.MediaType}' is not specified");