API: issues documentation and API functions updated

The issues documentation is updated with infos to status codes and the deprecated `DELETE` function and
how to close an issue. A few more tests added to check status codes of API functions.

lib/api/issues.rb

@@ -48,6 +48,7 @@ module Gitlab
       # Example Request:
       #   POST /projects/:id/issues
       post ":id/issues" do
+        bad_request!(:title) unless params[:title].present?
         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