Removed unnecessary stream seek in SetContent

- when running on the Microsoft Owin TestServer this was throwing as their
  resopnse stream is not seekable.

PactNet/Mocks/MockHttpService/Nancy/MockProviderAdminRequestHandler.cs

@@ -111,7 +111,6 @@ private Response GenerateResponse(HttpStatusCode statusCode, string message)
         private void SetContent(string content, Stream stream)
         {
             var contentBytes = Encoding.UTF8.GetBytes(content);
-            stream.Position = 0;
             stream.Write(contentBytes, 0, contentBytes.Length);
             stream.Flush();
         }