only return true in Union::is*Type* if there is a single type

[orklah]

This fixes a inconsistency I noticed. Union has two sets of useful functions: has*Type* and is*Type*. has ones checks that the type is in there while is ones checks that the only type(s) in the union are of the given type.

Before the fix, it allowed for isEmptyMixed or isEmpty to return true for empty-mixed|2 and empty|2 for example. It's quite unlikely this lead to actual bugs because those types are pretty rare, especially in Union with other types, but I didn't want to leave this like that :)