Callers who invoke `*http.Client.Do` with a nil *Client will now panic
at the top of c.Do, instead of panicking when `deadline` attempts to
read `c.Timeout`.
Errors inside of net/http can be difficult to track down because the
caller is often invoking the standard library code via an SDK. This
can mean that there are many places to check when code panics, and
raises the importance of being clear about error messages.
If nil receiver calls panic during the `deadline()` call, callers
may confuse the error with a more common timeout or deadline
misconfiguration, which may lead a caller who passed a nil receiver
(the author, for example) down the wrong rabbit hole, or cause them to
suspect their timeout/deadline logic. It is less common to configure
client.Jar, so the probability of detecting the actual problem, given
the underlying error cause, is higher.