simple_httpclient: handle connect_timeout or request_timeout of 0

simple_httpclient: handle connect_timeout or request_timeout of 0

Using a connect_timeout or request_timeout of 0 was effectively
invalid for simple_httpclient: it would skip the actual request
entirely (because the bulk of the logic was inside "if timeout:").
This was not checked for or raised as an error, it just behaved
unexpectedly.

Change simple_httpclient to always assert these timeouts are not None
and to support the 0 value similar to curl (where request_timeout=0
means no timeout, and connect_timeout=0 means curl default 300 seconds
which is very very long for a tcp connection).