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/issues.rb

@@ -48,7 +48,7 @@ module Gitlab
       # Example Request:
       #   POST /projects/:id/issues
       post ":id/issues" do
-        bad_request!(:title) unless params[:title].present?
+        required_attributes! [:title]
         attrs = attributes_for_keys [:title, :description, :assignee_id, :milestone_id]
         attrs[:label_list] = params[:labels] if params[:labels].present?
         @issue = user_project.issues.new attrs