When trying to use OAuth 2 authentication process it fails with error code 400 and the following message:

Client credentials missing; this request needs to be authenticated with the OAuth client id and secret

I am sending my request exactly as the RFC says. I took my github implementation and changed the Authorization Url and Access Token Url.
I am passing Client ID and
Client Secret correctly and
setting Content-Type: ''application/x-www-form-urlencoded"

I am being redirected to log-in page at bit-bucket but then when i perform the access_token request i get this error!

This is not working!