add ifdef for IsValueType on core clr

Author: gmarz

src/Nest/Search/Percolator/RegisterPercolator/RegisterPercolatorJsonConverter.cs

@@ -3,6 +3,7 @@
 using System.Linq;
 using Newtonsoft.Json;
 using Newtonsoft.Json.Linq;
+using System.Reflection;
 
 namespace Nest
 {
@@ -49,14 +50,14 @@ public override void WriteJson(JsonWriter writer, object value, JsonSerializer s
 				foreach(var kv in request.Metadata)
 				{
 					writer.WritePropertyName(kv.Key);
+#if DOTNETCORE
+					if (kv.Value.GetType().GetTypeInfo().IsValueType)
+#else
 					if (kv.Value.GetType().IsValueType)
-					{
+#endif
 						writer.WriteValue(kv.Value);
-					}
 					else
-					{
 						serializer.Serialize(writer, kv.Value);
-					}
 				}
 			}