Add support for file_link_data in file creation requests

[ob-stripe]

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

Add support for file_link_data in file creation requests.

The implementation is a bit of a hack: it uses form to flatten and stringify the contents of file_link_data, then parses the resulting query string to add the key/value pairs to the multipart buffer.

I've manually checked the contents of the request:

--554f676cf198d9e32740e881eb8cf161d311586da9b3cceaea7fcbe06135
Content-Disposition: form-data; name="purpose"

dispute_evidence
--554f676cf198d9e32740e881eb8cf161d311586da9b3cceaea7fcbe06135
Content-Disposition: form-data; name="file"; filename="test_data.pdf"
Content-Type: application/octet-stream

<snip>

--554f676cf198d9e32740e881eb8cf161d311586da9b3cceaea7fcbe06135
Content-Disposition: form-data; name="file_link_data[metadata][foo]"

bar
--554f676cf198d9e32740e881eb8cf161d311586da9b3cceaea7fcbe06135
Content-Disposition: form-data; name="file_link_data[create]"

true
--554f676cf198d9e32740e881eb8cf161d311586da9b3cceaea7fcbe06135--

[brandur-stripe]

Thanks for the fix!