AbstractGrant.toMap hardcode the grandType to Client_Credentials

AbstractGrant.toMap method has this line:
map.putSingle(OAuthConstants.GRANT_TYPE, OAuthConstants.CLIENT_CREDENTIALS_GRANT);

When the subclass, ResourceOwnerGrant overides this toMap, it calls the super.toMap(), which will cause this wrong value being added.

So, I think we should fix this by calling map.putSingle(OAuthConstants.GRANT_TYPE, getType()).
This way it will work for all 4 subclasses and will automatically work for future new class. Another way is to fix the ResourceOwnerGrant by not calling super.toMap.