Compatibility fix after HPKP was introduced

Firefox 35 changed some internal interfaces
and pushed down a check that the header
comes from a secure connection.

http://dxr.mozilla.org/mozilla-central/source/security/manager/boot/src/nsSiteSecurityService.cpp#407
https://hg.mozilla.org/mozilla-central/rev/aeacfe204b8d
https://bugzilla.mozilla.org/787133