Don't serialise "params" property of a request if it's null (#25).

Author: tintoy

src/JsonRpc/Client/Request.cs

@@ -12,6 +12,7 @@ public class Request
 
         public string Method { get; set; }
 
+        [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
         public object Params { get; set; }
     }
-}
+}