Adding _from_bytes helpers as a foil for _to_bytes.

[dhermes]

@nathanielmanistaatgoogle This is the child of #157.

I labeled it don't merge because I essentially just looked around for .decode and tried to preserve a lot of the stuff from the other PR, but haven't written any new tests.

Let's discuss (and I'll iterate) before merging.

UPDATE: I should note that I rebased the original PR if anyone wants to have a look: https://github.com/dhermes/oauth2client/tree/pr-157-rebased

Fixes #239 (but no tests yet tested by AssertionCredentialsTests.test_refresh_success_bytes)

[dhermes]

I should also FYI to @saaros that this is happening

[nathanielmanistaatgoogle]

Looks mostly good; what else do you want to do here?

[dhermes]

@nathanielmanistaatgoogle Sent the new commit.

I was looking for places that might need testing but kind of realized that the whole reason we test _from_bytes with all inputs is so that we don't have to send each of those types through every single code path which hits _from_bytes.

Also note I massively updated the Credentials.new_from_json method but left s as the only argument. This is because it is part of the public interface, so people can execute

Credentials.new_from_json(s=file_obj.read())

[nathanielmanistaatgoogle]

Looks good to me - anything remaining to be done here before merge? Anyone else who should review this?

[dhermes]

No and no. Merging.