Add API to create a file upload

[joshsmith]

We need to write an interface to file uploads. We can just do create for now:

API docs: https://stripe.com/docs/api/curl#create_file_upload

[snewcomer]

I can take a wack at this.

[snewcomer]

snewcomer@26e0197

So what are we using for testing. Still trying to figure that out?

[joshsmith]

@snewcomer can follow progress on code-corps/stripity_stripe_testing#1 but @DavidAntaramian is trying to have some time free up on this one.

For now, just test manually and push.

[joshsmith]

We won't be using VCR or anything like that. The interface has changed some for 2.0, too.

[snewcomer]

WIP
snewcomer@58622f6

One last thing is to get the header right with multipart/form-data

[joshsmith]

@snewcomer awesome! What's the reason for the header?

[snewcomer]

@joshsmith looks like the request type needs a Content-Type of multipart/form-data

https://stripe.com/docs/api#create_file_upload

I think this line (https://github.com/code-corps/stripity_stripe/blob/2.0/lib/stripe.ex#L229) needs to also have the option of merging in a different Content-Type multipart/form-data as well. Lmk if that is not the case.

[joshsmith]

@snewcomer you're right, in which case it might make sense to have a different kind of request here for the create. See how there's a different oauth_request? Think there should be one for multipart form data.

[joshsmith]

@snewcomer this especially makes sense because the base URL differs, too: https://uploads.stripe.com instead of the normal API URL.

[joshsmith]

Closed by #160.