title: Swashbuckle Swagger C# WebApi Core 2.0 - "Produces" is empty
body: <p>I'm having trouble finding the mistake.
  I've set up a little Web API with .net Core 2 and Swagger.
  After I added XML support (.AddXmlDataContractSerializerFormatters();), swagger doesn't show "Response Content Type" option in the UI.</p>
  
  <p>I also set "[Produces("application/json", "application/xml")]" as decorator.
  And yet the generated json always prints:</p>
  
  <pre><code>"consumes":[  
  
  ],
  "produces":[  
  
  ],
  </code></pre>
  
  <p>So I'm not sure what i'm doing wrong here.</p>
  
  <pre><code>    [HttpGet("Strom/{plz}")]
      [Produces("application/json", "application/xml")]
      public IActionResult GetStrom(string plz)
      {
          int iplz = 0;
  
          if (plz.Length != 5 || !int.TryParse(plz, out iplz))
          {
              return BadRequest("Die Postleitzahl ist ungültig.");
          }
          return Ok(GetOrte(plz, 1));
      }
  </code></pre>