Sparse fields and includes do not play well when applied together

[EugeneKostrikov]

Given all references are set up correctly:
GET '/people/:id?include=pets&fields=name,pets' will apply ?fields to both "people" and "pets" resources
GET '/people/:id?include=pets&fields=name' will not include "pets" at all

There might be other edge cases that need some treatment as well

[EugeneKostrikov]

The tests here illustrate the problem #157

[chiragdoctor]

Please find the PR for this issue #174 here for review.