Fix Brain.FM od Adam Williams

This is an add-on for use with https://brain.fm which is currently incompatible with newer versions of Firefox at the time of writing (2016-12-28). The reason for this incompatability is that there is an application/x-gzip content type sent by the server for the templates endpoint.

This add-on intercepts the response to set the content type to text/html, which makes the application work.