Fix assertion failure message meaning

[tanoabeleyra]

If the assertion fails it is because the "Content-Type" header is NOT "application/json"

[snoek09]

Thanks @tanoabeleyra.

I've looked into this and it seems this was done intentionally; #6736

Status: works for me.

[ogizanagi]

I think it was totally intended: the assertion message should generally describe the test case it covers, not the reason it fails.

I.e: The the "Content-Type" header is "application/json" assertion failed.

See NullOutputTest for instance.

[tanoabeleyra]

My apologies, I didn't know it was a common practice to write positive assert messages.

Thank you both for explaining it!