Added oauth_callback=oob to the request as request gets redirected to callback url set in the twitter app instead of returning the pin.

[YasKuraishi]

...t-of-band) authentication

Without oauth_callback param twitter does not return a pin but redirects to the callback url set in the app on twitter.com, hence failing the authentication. More at: https://dev.twitter.com/docs/auth/pin-based-authorization