Return base class from createFromJsonString

[OGKevin]

Steps to reproduce:

1. Create a class from JSON string by using the method createFromJsonString

What should happen:

1. The return value is casted to the subclass.

What happens:

1. The return value is casted to BunqModel.

Logs

• Logs No logs

Extra info:

• Tested on 0.12.3

Reference

• Return base class from createFromJsonString sdk_php#73

[OGKevin]

This is not the case in Python SDK 🙅‍♂️