The machinery for serializing links has been modified so that its

behvaiour can be changed by subclasses. The get_link_serializer

method takes a field name and returns the appropriate serializing

function for the link. The default function is serialize_link but

subclasses may provide a serialize_fieldname_link method that will be

used instead.

Tested the serialize_fieldname_link behaviour with a resource. It

worked as intended.
Ran unit tests.