Only parse jwk data as json when passing in a string

[belak]

This allows a user to request the /.well-known/jwks.json file from a server, parse it, look up the appropriate key, and load it without having to dump the key to json again.

I realize this will be superseded by version 2.0 as that introduces better support for JWKs in general, but this fixes the given use case for us until that releases.

[coveralls]

Pull Request Test Coverage Report for Build 149

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at ?%

---

Totals
Change from base Build 132:	0%
Covered Lines:
Relevant Lines:	0

---

💛 - Coveralls

[belak]

It looks like the only failing build is related to python 3.8 support with PyCrypto (due to time.clock() being removed). It's the same failure as master.

[belak]

This is most likely superceded by #470 which greatly improves on the JWK support.