Multiple SMS messages sent with a single instance of the client fail

[SteppingHat]

When using a single instance of the client across multiple requests, any SMS messages sent after the first message fail.

{"level":"debug","REST CLIENT":"Do","time":"2024-06-20T18:37:00+10:00","message":"Sending POST request to https://api.smsglobal.com/v2/sms"}
...
{"level":"debug","REST CLIENT":"Do","time":"2024-06-20T18:38:00+10:00","message":"Sending POST request to https://api.smsglobal.com/v2/sms/sms"}
...
{"level":"debug","REST CLIENT":"Do","time":"2024-06-20T18:39:00+10:00","message":"Sending POST request to https://api.smsglobal.com/v2/sms/sms/sms"}

This seems to be due to incorrectly mutated state, specifically the path, where appending "/sms" is done for each request, irrespective of it's currently stored value.

A workaround for this is to specifically set reset BaseURL before each request.

[SteppingHat]

I've opened PR #1 to address this issue