chore($httpBackend): preserve original non-zero http status code for file:// apps

Previously if an app was running from file:// origin we would always return either
http 200 or 404 depending on whether the response was present.

This changes the behavior so that we do this only if the protocol of the request
(not the origin) is file:// and only if the status code is 0.

Closes #4436
Closes #4587
Closes #4514