Fix attribute accessor as module_function warning

This code was introduced in 1354b07.

`module_function` cause the next methods to be defined as both
class methods and private instance methods, but the class method
part is never used, hence directly declaring them as private
is simpler and avoid the warning.

lib/googleauth/helpers/connection.rb

@@ -22,7 +22,7 @@ module Auth
     module Helpers
       # Connection provides a Faraday connection for use with Google::Auth.
       module Connection
-        module_function
+        private
 
         attr_accessor :default_connection