API: changed status codes for project hooks functions

Different status codes in the API lib are returned on hook creation, update or deletion.
If a required parameter is not given (e.g. `url` in `/projects/:id/hooks/:hook_id`) status
code 400 (Bad request) is returned. On hook deletion a 200 status code is returned, regardless if
the hook is present or not. This makes the DELETE function an idempotent operation. Appropriate tests
are added to check these status codes.

lib/api/projects.rb

@@ -195,11 +195,14 @@ module Gitlab
       #   POST /projects/:id/hooks
       post ":id/hooks" do
         authorize! :admin_project, user_project
+
+        error!("Url not given", 400) unless params.has_key? :url
+
         @hook = user_project.hooks.new({"url" => params[:url]})
         if @hook.save
           present @hook, with: Entities::Hook
         else
-          error!({'message' => '404 Not found'}, 404)
+          not_found!
         end
       end
 
@@ -215,7 +218,7 @@ module Gitlab
         @hook = user_project.hooks.find(params[:hook_id])
         authorize! :admin_project, user_project
 
-        error!("Url not given", 400) if !params.has_key? :url
+        error!("Url not given", 400) unless params.has_key? :url
 
         attrs = attributes_for_keys [:url]
         if @hook.update_attributes attrs
@@ -234,8 +237,13 @@ module Gitlab
       #   DELETE /projects/:id/hooks
       delete ":id/hooks" do
         authorize! :admin_project, user_project
-        @hook = user_project.hooks.find(params[:hook_id])
-        @hook.destroy
+        error!("Hook id not given", 400) unless params.has_key? :hook_id
+
+        begin
+          @hook = ProjectHook.find(params[:hook_id])
+          @hook.destroy
+        rescue
+        end
       end
 
       # Get a project repository branches