In Rails, `ActionDispatch::Http::UploadedFile` has an accessor to access
a `Tempfile` object. However, `Rack::Test::UploadedFile` does not
provide that, but provinding the `#path` method instead. To be able to
support both, we have to check for existance for `#tempfile` method,
then fallback to `#path` method.

Fixes #807