Add spec for base64:encode_binary/1

[Paul Oliver]

Dialyzer was reporting that use of base64:encode(Foo :: binary()) had spec
binary() -> <<_:_*1>>.  The binary comprehension in base64:encode seems to
result in the return type of <<_:_*1>>.  The patch below enforces a return
type of binary().

git fetch git://github.com/puzza007/otp.git base64_encode_binary_spec

Cheers,
Paul.