Fix caching for namespaced polymorphic associations

[alachaum]

Hi guys,

I noticed there was an issue with caching when looking up namespaced resource classes for polymorphic associations. This issue prevents include queries on resources having a polymorphic association.

Something like this:

class Api::V1::PictureResource < JSONAPI::Resource
  caching
  has_one :imageable, polymorphic: true
end

class Api::V1::ProductResource < JSONAPI::Resource
  has_one :picture
end

..will fail when doing:

# Fails because JR attempts to lookup PictureResource when compiling cached fragments
GET /api/v1/pictures?include=imageable

The test attached to this PR does not create a 500 if the patch is not applied (because both PictureResource and Api::V1::PictureResource are defined) but clearly show that the links are incorrect when caching is enabled.

[lgebhardt]

@alachaum I think this has been fixed with the rework that has happened in #1045. Please check it out and reopen this if it's still a problem. We'll need to fix it another way since the to_real_resource method no longer exists.