Fix PIN authorization request

When I tried running poauth.rb, I got this error

    /Library/Ruby/Gems/2.0.0/gems/oauth-0.4.7/lib/oauth/consumer.rb:216:in `token_request': 403 Forbidden (OAuth::Unauthorized)
      from /Library/Ruby/Gems/2.0.0/gems/oauth-0.4.7/lib/oauth/consumer.rb:136:in `get_request_token'
      from poauth.rb:32:in `<main>'

After updating it to use the header auth scheme, it worked properly.