Tests overriding of FakeRequest Content-Type header

The Content-Type in the request was being overridden by a mixed order merge.
The actual problem was addressed in an earlier version of this commit, but
was addressed in #3119 before this commit passed legal review.

Fixes #3019.
Merge pull request #3207 from colindean/reverse-header-map-gh3019

Fixes overriding of FakeRequest Content-Type header