API: extracted helper method to validate required parameters, code clean up

Added a helper method to check if required parameters are given in an API call. Can be used
to return a `400 Bad Request` return code if a required attribute is missing.
Code clean up and fixed tests.

lib/api/groups.rb

@@ -29,9 +29,7 @@ module Gitlab
       #   POST /groups
       post do
         authenticated_as_admin!
-
-        bad_request!(:name) unless params[:name].present?
-        bad_request!(:path) unless params[:path].present?
+        required_attributes! [:name, :path]
 
         attrs = attributes_for_keys [:name, :path]
         @group = Group.new(attrs)