Contradictory terminology about native types

Currently `future.utils.native_str` is the default native string type.

One would therefore conclude that `future.utils.bytes_to_native_str` was the correct way to turn `newbytes` into a `native_str`, at least on Python 2. This does not work, as it is in fact a no-op on Python 2.

Personally this could be solved with a hint in the docstring (and a pointer to use `native`), but it could also be seen as a naming or coding bug.

Thanks for the lovely work. :)