Fix for issue #8

[happycollision]

This PR is just to let you know I am working on this today. You may not want to version bump from the last merge until this one is done... assuming I get it right!

[happycollision]

This would produce a failure in tests that looks like this:

Failures:

  1) JsonapiSpecHelpers#assert_payload when payload contains a "relationship-only" item still properly throws when no item is present
     Got 2 failures from failure aggregation block "payload contains an id and type".
     # ./lib/jsonapi_spec_helpers.rb:57:in `assert_payload'
     # ./spec/assert_payload_spec.rb:278:in `block (4 levels) in <top (required)>'

     1.1) Failure/Error: expect(json['id']).to eq(record.id.to_s)

            expected: "1"
                 got: nil

            (compared using ==)
          # ./lib/jsonapi_spec_helpers.rb:59:in `block in assert_payload'

     1.2) Failure/Error: expect(json['jsonapi_type']).to_not be_nil

            expected: not nil
                 got: nil
          # ./lib/jsonapi_spec_helpers.rb:62:in `block in assert_payload'

[happycollision]

I am not 100% sure of the ramifications of ids not being present (when you are creating a record) but I think that wouldn't be a problem since you wouldn't be testing your input for a create action.

[happycollision]

If you want to merge this without the first commit (since you just merged my last PR), I am happy to rebase.