Tell providers to return json on token request.
Tell providers to return json on token request.

jsfan/requests-oauthlib,singingwolfboy/requests-oauthlib,dongguangming/requests-oauthlib,dasevilla/requests-oauthlib,abhi931375/requests-oauthlib,sigmavirus24/requests-oauthlib,elafarge/requests-oauthlib,gras100/asks-oauthlib,jayvdb/requests-oauthlib,requests/requests-oauthlib,jayvdb/requests-oauthlib,dasevilla/requests-oauthlib,lucidbard/requests-oauthlib