Convert headers coming from requests to dict before serializing (bug 941000)

https://bugzilla.mozilla.org/show_bug.cgi?id=941000

requests changed the way they return headers recently, so when we upgraded it broke the reviewer tools since it serializes those headers. We were mocking with a plain dict in our tests so we didn't see that. See https://github.com/kennethreitz/requests/issues/1380 for more information.