[FR] Add test for "must not exceed the maximum of 4 weeks"

**Is your feature request related to a problem? Please describe.**
When inserting a pushnotification, its TTL may not exceed 4 weeks according to docs.
This is only checked after the request is send to the API. It should be checked in the library, to easier catch bugs and not annoy the API.

**Describe the solution you'd like**
Check the payload TTL locally (in the library) for common mistakes.

**Describe alternatives you've considered**
Right now its done to the enduser of checking this. And the results may vary. In the end it can happend due to bugs or other things that invalid payloads still get send to the API - when this library could've prevented that.

Thanks for consideration.