Fix multipart encoding of nested parameters

[ob-stripe]

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

This one was a bit tricky. stripe-python rolls its own multipart encoder, which only supported two types of values: file-like (i.e. an object that has a read attribute) or strings. All other types caused an exception to be raised.

So there were actually two different issues to fix:

1. the parameters need to be flattened (which solves the problem for compound types -- lists and dicts are gone after this step)
2. handle non-string scalar types (booleans, integers, etc.)

The first issue was easily solved by calling api_requestor._api_encode() in MultipartDataGenerator to flatten the parameters.

To solve the second issue, I changed util.utf_8() to stringify all non-file-like values. This could potentially have side effects for regular form-encoding, but I'm fairly sure that's not the case: form-encoding works by passing the dictionary of flattened params to urlencode which would stringify non-string values anyway.

The second issue is solved by stringifying non-file-like values before adding them to the multipart buffer.

[remi-stripe]

To solve the second issue, I changed util.utf_8() to stringify all non-file-like values. This could potentially have side effects for regular form-encoding, but I'm fairly sure that's not the case: form-encoding works by passing the dictionary of flattened params to urlencode which would stringify non-string values anyway.

Could we special-case this just for file upload? My inner paranoia is tingling right now with that sentence :p

[ob-stripe]

Not sure why the PR is failing on Travis for some Python versions -- I suspect a circular import issue, but I can't reproduce locally. I'll try shuffling some imports around anyway.

EDIT: Fixed.

[ob-stripe]

@remi-stripe I've updated the implementation to only stringify values in MultipartDataGenerator.

[remi-stripe]

Thanks for making the change. This looks great to me and I like that you explicitly tested a nested dict!

[brandur-stripe]

Thanks for looking at this, and awesome tests! LGTM.

[ob-stripe]

Thanks both!

[ob-stripe]

Released as 2.24.1.