Fix validation when passing an interface to the injects list.

I admit that this fix is clumsy. The core problem is that
the 'injects' list serves double duty: it includes types that
we can call ObjectGraph.get() on (could be interfaces) and also
types that we call ObjectGraph.inject() on (can't be interfaces).

The right fix is to rethink the way Dagger internally tracks
and links these keys.
Merge pull request #222 from square/jwilson/inject_interface

Fix validation when passing an interface to the injects list.