Switch from Basic to Bearer auth

Stripe will be supporting Basic auth for the indefinite future, but
our API keys map onto Bearer auth more cleanly, and using it lets us
get rid of the base64 encoding step.

lib/stripe.rb

@@ -554,12 +554,12 @@ module Stripe
     end
 
     headers = {
-      :user_agent => "Stripe/v1 RubyBindings/#{Stripe::VERSION}"
+      :user_agent => "Stripe/v1 RubyBindings/#{Stripe::VERSION}",
+      :authorization => "Bearer #{api_key}"
     }.merge(headers)
     opts = {
       :method => method,
       :url => url,
-      :user => api_key,
       :headers => headers,
       :open_timeout => 30,
       :payload => payload,