Cannot nullify model fields using API

Summary

It is not possible to set a model's field to nil using the API. This affects all the endpoints where API::Helpers#attributes_for_keys is used.

Steps to reproduce

See a failing test case https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5696

Possible fixes

This is easy to fix from the point of view of the changes needed, but it may break some clients of the API. Clearly this is a bug, but people may rely on this bug in their code.

/cc @smcgivern