Enable gzip encoding on commonplace URLs (bug 987403)

https://bugzilla.mozilla.org/show_bug.cgi?id=987403

We know those pages are not going to contain any secrets, so we can safely use gzip without fear of BREACH attacks here even though they are served through SSL.
Would it be good to assert that gzip was called? All these tests check for 200 and content-encoding but not the content itself.