I have reduced the issue to the simplest possible case.
I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)
The default request header 'Accept' on many browsers is something like Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8. This means that currently, if the response is not text/html, then the response Content-Type incorrectly contains the q= parameter. (According to this, the q parameter specifies which content-types the browser prefers).
More than happy to reopen this if you can demo an example against http://restframework.herokuapp.com/ that I can replicate, or if you can demo that the behavior has changed, and that the latest version of REST framework running the tutorial does demonstrate the issue, or if you can provide an example pull request that demonstrates that we do have an issue.