6ca709816f
Validate the content-type of the response when fetching an object, according to https://www.w3.org/TR/activitypub/#x3-2-retrieving-objects. content-type headers had to be added to many mocks in order to support this, some of this was done with a regex. While I did go over the resulting files to check I didn't modify anything unrelated, there is a possibility I missed something. Closes pleroma#1948