Add second, optional argument to UploadIO#respond_to? method signature

This pull request changes the method signature for the `UploadIO#respond_to?` method to correctly include the second, optional argument. Unfortunately this omission was causing issues on Rubinius master, as reported by @craiglittle in the discussion for [this commit](https://github.com/rubinius/rubinius/commit/9fa7e49).

While this has since been fixed in Rubinius, it doesn't change the fact that the `respond_to?` method was being overridden incorrectly to begin with.