Emulator should not need content-type header, or provide proper error response
The real PubSub API request a publish request (potentially others) without the content-type header.
The PubSub Emulator returns a 404 error if no content-type header is set, or a wrong content-type header is set.
404 is the wrong HTTP status code in such a situation (400 is correct), and without a proper error message, this is extremely difficult to debug. I wasted a lot of time debugging whether I set up the topic correctly, and whether I constructed the URL correctly.
Please either ignore the content-type header (like the PubSub API does), or provide a 400 response with a proper error message.