Fix panic when parsing connection timeout

[sorairolake]

Panic if seconds of connection timeout is greater than Duration::MAX or not finite (see Duration::from_secs_f64), so fix this.

Note this can be simplified when Duration::try_from_secs_f64 is stabilized.

[blyxxyz]

Nice catch!

Two concerns:

• This solution rejects -0, while HTTPie accepts it. Do we want that?

• If the timeout is too large then maybe we should set it to Duration::MAX instead? That's still a couple billion years so the user won't be able to tell 🙂

[sorairolake]

If the concerns need to be solved, I will do it.

[blyxxyz]

Looks good. Maybe @ducaale has an opinion about the points from earlier but I don't think they're very important.

[ducaale]

Two concerns:

• This solution rejects -0, while HTTPie accepts it. Do we want that?
• If the timeout is too large then maybe we should set it to Duration::MAX instead? That's still a couple billion years so the user won't be able to tell 🙂

It would've been if the first point could be addressed but I don't see it as a blocker 👍