test_util: Fix logic in get_manifest_digests_missing
test_util: Fix logic in get_manifest_digests_missing

If there's no 'Content-Type' header, the responses module adds
Content-Type: text-plain. This was confusing the test code, and the checks
were made to pass in a way that didn't fully make sense.

Switch to directly mocking requests.get(), then fix up the logic
in the test.

maxamillion/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor,DBuildService/atomic-reactor,maxamillion/atomic-reactor,jarodwilson/atomic-reactor,DBuildService/atomic-reactor,vrutkovs/atomic-reactor,jarodwilson/atomic-reactor,fr34k8/atomic-reactor,vrutkovs/atomic-reactor,projectatomic/atomic-reactor