fix($httpBackend): fixup patch for #9979

Builds ontop of c480462 from #9979
Considering `undefined` is used throughout the code (in fact I'be never seen any `void 0` occurrence in the core), this just introduces inconsistency and leaves maintainers wonder what's special about this case.
I am very surprised that this suggestion came from a maintainability advocate like you :smiley:

We should either change all occurrences to `void 0` or none.
And considering that there are already meassures taken against rebinding `undefined`, I think using `undefined` is pretty safe.