Pass Link headers in to Serializer

Problem/Motivation

Link Headers can be used to add relations to a message independent of the format used. For example, Link: <http://ex.org/rest/types/node/article>; rel="type"

Currently, there is no way to access the information from those headers once you are serializing/deserializing a request.

Proposed resolution

In RequestHandler, extract the Link headers from the request and pass them in to the Serializer using the $context.