bugfix: fixed assorted issues with serializer and view

Related to applications/hubl#905 (closed) and applications/knowledge-base/knowledgebase-front#258 (closed)

The issue was that external resources should always be rendered as having only an @id and an @type, not just on GET requests

Fixed issue where @type was not being rendered on objects/fields in certain scenarios

Fixed another issue found in the cache tests where a request was causing the serializer to hang

Fixed issue where foreign keys were being serialized with the wrong urlid (#293 (closed) )

Extended tests everywhere

Closes #308

Closes #293 (closed)

Opened a whole can of worms making this change! On the bright side though I was able to fix some other issues in the process 😄

@balessan could I have some budget to review all of the unit tests please and write any more which I can think of? The serializer in particular seems a little fragile