Add mime type (#531)

Author: EatonZ

src/ServiceStack.Text/HttpUtils.cs

@@ -1384,6 +1384,7 @@ public static class MimeTypes
         public const string Xml = "application/xml";
         public const string XmlText = "text/xml";
         public const string Json = "application/json";
+        public const string ProblemJson = "application/problem+json";
         public const string JsonText = "text/json";
         public const string Jsv = "application/jsv";
         public const string JsvText = "text/jsv";