Fixed bug that caused the async RestClient callbacks to never get called.

When an exception was thrown from a RestRequest.OnBeforeDeserialization
event handler, async callbacks were not called. When using the Task-based
Execute methods, this resulted in the TPL Task never completing.
Added the request to the RestResponse instance when building an error response.
Merge pull request #516 from mehdime/master

Fixed bug that caused the async RestClient callbacks to never get called...