Fix Resource Owner Password Authentication Flow

Created by: roshangautam

As per the documentation here: https://github.com/doorkeeper-gem/doorkeeper/wiki/Using-Resource-Owner-Password-Credentials-flow to make "password" type grant_flow work once should add Doorkeeper.configuration.token_grant_types << "password" after Doorkeeper.configure do block

I have also added a check against username field such that OAUTH can be used with both email and username field.