Reject requests without X-Hub-Signature when needed

Summary:
A request to /gate/GitHub/<service with secret defined> must has
a X-Hub-Signature header, so we can directly consider not legit
those without.

This fixes the following exception:

ErrorException in XHubSignature.php line 83:
hash_equals(): Expected user_string to be a string, null given

Test Plan: Fire a Phabricator payload to /gate/GitHub/<door with secret>

Reviewers: dereckson

Subscribers: alken-orin

Maniphest Tasks: T695

Differential Revision: http://devcentral.nasqueron.org/D260