Use Types.isSameType for comparing TypeMirrors.

The documentation on TypeMirror.equals states "Semantic comparisons of type equality should instead use Types.isSameType(TypeMirror, TypeMirror). The results of t1.equals(t2) and Types.isSameType(t1, t2) may differ.".
Merge pull request #433 from square/jw/types-comparison

Use Types.isSameType for comparing TypeMirrors.