REST docs use magic accept header values
Steps to reproduce
When testing the External API extensions, I've run into the problem that requests of the REST docs failed because they set "text/plain" in the HTTP request header "accept".
A closer look revealed that the values of the accept header are quite magically determined by JQuery's ajax function.
I've seen various things like ": text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,/;q=0.8".
The goal here is to have a deterministic accept header value so that development doesn't get slowed down because JQuery infers whatever incompatible accept header (just because of the REST docs)