Allow to have a custom auth header name

lib/rails_api_authentication/authable.rb

@@ -96,11 +96,17 @@ module RailsApiAuthentication
       end
 
       def auth!(request)
-        token = request.env["HTTP_#{self.to_s.upcase}_TOKEN"] || request.env["#{self.to_s.upcase}_TOKEN"]
+        token = request.env["HTTP_#{token_key}_TOKEN"] || request.env["#{token_key}_TOKEN"]
         user = auth(token)
         user.nil? ? raise(UserError.new(401, '-1', 'Unauthorized')) : user
       end
 
+      attr_writer :token_key
+
+      def token_key
+        @token_key ||= self.to_s
+      end
+
       def register(name, password, attrs={})
         raise(UserError.new(400, '-1', 'password is blank')) if password.blank?
         valid! name, attrs.delete(@valid_key)