title: Why does Spray error "could not find implicit value for parameter marshaller"?
body: <p>I use a <code>spray-json</code> for convert some case class into json, but i have  a troubles with convert List to json:</p>
  
  <pre><code>case class Foo(id: Int)
  
  object FooJsonSupport extends DefaultJsonProtocol with SprayJsonSupport {
    implicit val fooFormats = jsonFormat1(Foo)
  }
  
  //and i have a list with Foo:
  val list = List(Foo(1), Foo(2), Foo(3))
  
  // and i want to return json response
  
  post {
    respondWithMediaType(`application/json`) {
      complete(list)
    }
  }
  </code></pre>
  
  <p>After I run the sample, I get the error:</p>
  
  <blockquote>
    <p>could not find implicit value for parameter marshaller: spray.httpx.marshalling.ToResponseMarshaller[scala.collection.immutable.List[Post]]</p>
  </blockquote>
  
  <p>How to fix it?</p>