WithProxy(...) also use all proxy settings (#550)

src/WireMock.Net/Serialization/JsonSerializationConstants.cs

@@ -0,0 +1,19 @@
+using Newtonsoft.Json;
+
+namespace WireMock.Serialization
+{
+    internal static class JsonSerializationConstants
+    {
+        public static readonly JsonSerializerSettings JsonSerializerSettingsDefault = new JsonSerializerSettings
+        {
+            Formatting = Formatting.Indented,
+            NullValueHandling = NullValueHandling.Ignore
+        };
+
+        public static readonly JsonSerializerSettings JsonSerializerSettingsIncludeNullValues = new JsonSerializerSettings
+        {
+            Formatting = Formatting.Indented,
+            NullValueHandling = NullValueHandling.Include
+        };
+    }
+}