Accept JSON-encoded requests

[bbockelm]

Currently, we only accept form-encoded requests. Several users have tripped up on this, assuming the application will also accept JSON-encoded requests if the correct content-type was sent.

Here's what someone recently tried:

curl -L -k --key ~/.globus/userkey.pem --cert ~/.globus/usercert.pem -v -X POST -H "Content-Type: application/json" -H "Accept: application/json" -d '{"grant_type": "client_credentials"}' https://cms.scitokens.org/token