Fixed type signature of uninhabited method.

Added unit test to prevent similar mistakes from happening again. The
previous method was wrong because it dereferenced a pointer to a void type to
match on the result. No self pointer was needed, and the correct method
signature took the self value by value.
auto merge of #6442 : sstewartgallus/rust/incoming, r=pcwalton

Added unit test to prevent similar mistakes from happening again. The
previous method was wrong because it dereferenced a pointer to a void type to
match on the result. No self pointer was needed, and the correct method
signature took the self value by value.

I feel silly that I made this mistake in https://github.com/mozilla/rust/pull/6348