Remove ON_VALUE usage because hasAuthCallback is always set

[ben-xD]

Xcode was warning about an unused returned value when we used:

[co initWithClientOptions: o hasAuthCallback: value];

I just call the initializer directly and return the result.

Also, there was no need for the use of ON_VALUE macro. dictionary[TxClientOptions_hasAuthCallback] is always false or true, never nil or NSNull.

When you see

  _writeToJson(
        jsonMap, TxClientOptions.hasAuthCallback, v.authCallback != null);

This is saying, we only set the key/value pair to jsonMap if v.authCallback != null is non null. Well, != never returns null.

[ben-xD]

@lukasz-szyszkowski, I reverted the rename of variables (e.g. o to clientOptions) and have put this in a separate PR #262.