hg_http_controller: avoid masking some errors
In case the inner HTTParty call to hgserve didn't get to a conclusion (e.g., because of a timeout), hgserve response doesn't have its Content-Type set, and this ends up as:
ArgumentError (:type option required): app/controllers/projects/hg_http_controller.rb:79:in `hg_post'
because we try and return our own response without a proper Content-Type. In this changeset, we add a default Content-Type to avoid this masking.
If hgserve would itself have a bug leading it to incorrectly send us a response without a proper Content-Type, we would similarly not mask what happens with this ArgumentError
--HG-- branch : heptapod