Move request deserialization from Dispatcher to Content and Request.

Add the serialization engine to the Request object when built from within
the Context; rather than doing it later from the Dispatcher.

Deserialization then occurs when the Request object is instantiated.

Closes #369.
Merge branch 'veryrusty/request_deserialization' into devel

* veryrusty/request_deserialization:
  Move request deserialization from Dispatcher to Content and Request.