Support Tempfiles in file_upload creation requests

[ob-stripe]

r? @brandur-stripe
cc @stripe/api-libraries @remi-stripe

Fixes #610.

Back in #508, we reintroduced support for passing File instances, but not Tempfile instances. (I'm not sure why Tempfile is not a subclass of File but ¯\_(ツ)_/¯)

This PR adds support for passing Tempfile instances, which is useful for e.g. opening a remote file via open-uri.

[brandur-stripe]

Back in #508, we reintroduced support for passing File instances, but not Tempfile instances. (I'm not sure why Tempfile is not a subclass of File but ¯_(ツ)_/¯)

Haha, yes that does seem to really strange!

I just took a quick look at it, and it turns out that the reason is that it uses a special DelegateClass(File) as its parent. This presumably does the right thing, but has the side effect of rendering the class hierarchy nonsensical. Gotta love metaprogramming :)