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.