Provide interface for accessing underlying IO object

In some cases users may need to work with/manipulate more of the
Tempfile api than provided by Upload. Allow users to get at the
underlying io via the common to_io method of IO/IO-like objects
Merge pull request #14755 from timlinquist/to_io_http_upload

Use common to_io so users can access the underlying IO object
Implement to_io as an alias
Change the method description
Add CHANGELOG entry for #14755 [ci skip]