Allow uploading files from InputStreams and Files

[drake-stripe]

This allows users to create a FileUpload with either a File or a InputStream under the file parameter. It defaults to calling the uploaded file "blob".

#299

[brandur-stripe]

@drake-stripe Can you take a look at the failed build before we find you a reviewer? It looks like checkstyle is failing:

> Task :checkstyleMain
[ant:checkstyle] [ERROR] /home/travis/build/stripe/stripe-java/src/main/java/com/stripe/net/MultipartProcessor.java:75: Line is longer than 100 characters (found 102). [LineLength]
> Task :checkstyleMain FAILED

Also, this is a bit of a nit, but those of us who work on these external projects tend to try to keep slightly better commit hygiene than what you might see internally. It might be slightly better to get into a workflow where you pull minor changes/fixes into a single commit with git commit --amend, or use git rebase -i to fix a series of them. It's not the end of the world either way because we can squash them down before merging here, but it does save us from having to strip all the "fix test", "fix error", etc. cruft from commit messages.

Thanks for taking a crack at this!

[ob-stripe]

@drake-stripe Looks good overall, but you'll need to fix the style errors as Brandur mentioned above. You can run both the test suite and the linter locally with ./gradlew check (or ./gradlew checkstyleMain checkstyleTest if you just want to run the linter).

Also, could you add a test case in the functional test for FileUpload? You can copy the testCreate function into a new testCreateWithStream case.

[drake-stripe]

PR now passes linter, and there is a functional test implemented for InputStream uploading. I also squashed it into one commit with a more descriptive commit message.

[brandur-stripe]

Minor, but since these methods are meant to be internal anyway, can we just get rid of the File form of addFileField since it's only used in one place anyway? If other places in the library need to do this, they can instantiate their own FileInputStream.

[drake-stripe]

Done.

[brandur-stripe]

Left a minor comment, but thanks a lot for fixing all that stuff Drake!

(Olivier, you're a lot better learned in Java than I am, so I'll leave a final review to you.)

[ob-stripe]

Released in 5.48.0.