Fix unintended avatar reset

I like having an avatar after changing my username ^p^

src/Discord.Net/API/Rest/ModifyCurrentUserParams.cs

@@ -7,7 +7,7 @@ namespace Discord.API.Rest
     {
         [JsonProperty("username")]
         public Optional<string> Username { get; set; }
-        [JsonProperty("avatar"), Image]
+        [JsonProperty("avatar", NullValueHandling = NullValueHandling.Ignore), Image]
         public Optional<Stream> Avatar { get; set; }
     }
 }