Credentials.fromJson doesn't support underscored keys about oauth2

[stevenroose]

I just noticed that the handleAccessTokenResponse method in the src/handle_access_token_response.dart file parses the OAuth response and does support the underscore format. However, that method is not exposed.

I have to do a manual access token request (the AuthorizationGrant flow is not the one I need (Reddit public API)), so I have to build credentials from the token response manually. Exposure of that method or a separate Credentials.fromAccessTokenResponse would be very useful.

[nex3]

Credentials.fromJson isn't a method that interacts with the OAuth2 protocol; it and Credentials.toJson() are utility methods designed to make it easy for users to save the user's credentials somewhere and restore them later. The only thing the format is expected to interoperate with is the Dart oauth2 package.

Currently only the authorization grant flow is supported by this package. If you'd like to add a different flow, please feel free, but I'd rather not expose library-internal methods in order to partially support a different flow.