Set correct URL in tasks JSON builder

I want to use the tasks JSON builder to demonstrate using controller
tests for non-HTML requests and responses, but the scaffold-generated
builder in place now doesn't account for the fact that the tasks
resource is nested under projects. Update the builder to build the
correct route.

Author: ruralocity

app/views/tasks/_task.json.jbuilder

@@ -1,2 +1,2 @@
 json.extract! task, :id, :name, :project_id, :completed, :created_at, :updated_at
-json.url task_url(task, format: :json)
+json.url project_task_url([task.project, task], format: :json)