Braintree blue cardholder name

A user has reported within spree/spree_gateway#164 that the cardholder_name was not being passed in when using Braintree. Upon investigation, I have found that this appears caused by a missing parameter within active_merchant's `braintree_blue.rb` code. 

All the tests pass, and attempting to put a payment through using Spree using this modified version of active_merchant results in the "Cardholder Name" on Braintree no longer being blank.