title: ASP.NET Core HTTPRequestMessage returns strange JSON message
body: <p>I am currently working with ASP.NET Core RC2 and I am running into some strange results.
  So I have an MVC controller with the following function:</p>
  
  <pre class="lang-cs prettyprint-override"><code>public HttpResponseMessage Tunnel() {
      var message = new HttpResponseMessage(HttpStatusCode.OK);
      message.Content = new StringContent("blablabla", Encoding.UTF8);
      message.Content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("text/plain");
      message.Headers.CacheControl = new System.Net.Http.Headers.CacheControlHeaderValue {
          NoCache = true
      };
  
      return message;
  }
  </code></pre>
  
  <p>If I call this with postman with an Accept header set to text plain I get this response:</p>
  
  <pre class="lang-json prettyprint-override"><code>{
    "Version": {
      "Major": 1,
      "Minor": 1,
      "Build": -1,
      "Revision": -1,
      "MajorRevision": -1,
      "MinorRevision": -1
    },
    "Content": {
      "Headers": [
        {
          "Key": "Content-Type",
          "Value": [
            "text/plain"
          ]
        }
      ]
    },
    "StatusCode": 200,
    "ReasonPhrase": "OK",
    "Headers": [
      {
        "Key": "Cache-Control",
        "Value": [
          "no-cache"
        ]
      }
    ],
    "RequestMessage": null,
    "IsSuccessStatusCode": true
  }
  </code></pre>
  
  <p>I really do not understand how this is the generated reponse to the above controller. It is basically a JSON serialization of the entire message itself and does in no way contain the "blablabla" I intended to send.</p>
  
  <p>The only way I have gotten the desired result is by making my controller function return <code>string</code> instead of <code>HttpResponse</code>, but that way I am unable to set headers like <code>CacheControl</code></p>
  
  <p>So my question is: why do I get this strange response? It seems like very weird behaviour to me</p>