Adding HttpResponseException to WebApi Shim.

Fixes #1284
Well the extra assignment is not required, it would have done conneg based on the runtime type. Here though since the declared type is already known ( no need of doing an object.gettype), we provide it here. The purpose of the DeclaredType property is to represent just that -- The `declared type`, I don't think it would be right to automatically set it to the runtime type. Let me know if you think otherwise.