fix(client): ensure setter method updates both _base_url and client.base_url

[tomoish]

base_url of an OpenAI client instance cannot be changed by setter method.

This PR updates the setter method to change both _base_url and client.base_url.

Closes #913.

[RobertCraigie]

Thanks @tomoish! We actually want this fix to be slightly different and not set the base_url property on the underlying http client as well.

I've pushed a fix separately for this and marked you as a co-author, #919.