Fix bug where custom fields are not properly merged

[sudomabider]

I was experiencing 2 issues:

1. HTTP requests and console inputs are behaving differently for this particular scenario: custom fields get ignored for http request but are merged fine in the console. I think what happens is: Config is merged in the service provider at model resolution. But then merged again in the model constructor in a different way. It is also why the test is not catching the bug. The bug currently only exists for http requests (I didn't have time to dig deep enough to explain why but I think the model resolution callback has something to do with it).
2. In the Notificatoin model, getCustomFillableFields() flattens the additional fields array from the config file which forces it to be a numeric array. And then mergeFillable() uses array addition (+) which ignores the custom fields because the numeric keys already exist in the left-hand array.

[Gummibeer]

Hey, sorry for the long delay and thanks for your help! :)