fix($httpBackend): set headers with falsy values

This is a breaking change. To migrate to the new behavior,
delete or set headers to `undefined` to avoid having them sent.
To restore the old behavior, override `$httpBackendProvider`
with the old implementation.

Closes #2984
docs(ngRoute): add angularEvent param to $routeChangeStart event

Adds missing implied first argument, `angularEvent`, to match
the rest of the `$routeChange` event documentation.