Add pretty print support for null values in JsonObjects

mcs/class/System.Json/System.Json/JsonObject.cs

@@ -141,7 +141,13 @@ namespace System.Json
 				stream.WriteByte ((byte) '"');
 				stream.WriteByte ((byte) ',');
 				stream.WriteByte ((byte) ' ');
-				pair.Value.Save (stream);
+				if (pair.Value == null) {
+					stream.WriteByte ((byte) 'n');
+					stream.WriteByte ((byte) 'u');
+					stream.WriteByte ((byte) 'l');
+					stream.WriteByte ((byte) 'l');
+				} else
+					pair.Value.Save (stream);
 			}
 			stream.WriteByte ((byte) '}');
 		}

mcs/class/System.Json/System.Json/JsonValue.cs

@@ -148,7 +148,10 @@ namespace System.Json
 					w.Write ('\"');
 					w.Write (EscapeString (pair.Key));
 					w.Write ("\": ");
-					pair.Value.SaveInternal (w);
+					if (pair.Value == null)
+						w.Write ("null");
+					else
+						pair.Value.SaveInternal (w);
 					following = true;
 				}
 				w.Write ('}');