title: akka http download marshalling in akka http testkit
body: <p>I'm trying to unmarshal a csv download in my akka http test: </p>
  
  <pre class="lang-scala prettyprint-override"><code>val bseq: immutable.Seq[ByteString] = Await.result(response.entity.dataBytes.runWith(Sink.seq), 20 seconds)
          val str = bseq.map(_.utf8String).mkString
          logger.debug(s"res:${str}")
  </code></pre>
  
  <p>but I get: </p>
  
  <pre><code>akka.http.scaladsl.marshalling.NoStrictlyCompatibleElementMarshallingAvailableException: None of the available marshallings (List(WithFixedContentType(application/octet-stream,&lt;function0&gt;))) directly match the ContentType requested by the top-level streamed entity (text/csv; charset=UTF-8). Please provide an implicit `Marshaller[akka.util.ByteString, HttpEntity]` that can render akka.util.ByteString as [text/csv; charset=UTF-8]
      at akka.http.scaladsl.marshalling.LowPriorityToResponseMarshallerImplicits$$anonfun$fromEntityStreamingSupportAndByteStringSourceMarshaller$1$$anonfun$apply$5$$anonfun$4$$anonfun$6$$anonfun$apply$7.apply(PredefinedToResponseMarshallers.scala:117)
  </code></pre>
  
  <p>How can I unmarshall the csv file in my test case?</p>