Preserve invalid status code at best effort

[upsuper]

This should help on usecases mentioned in #144 without adding performance penalty for everyone else.

Theoretically with the original form, we can use NonZeroU16 and Rust would optimize Result<StatusCode, InvalidStatusCode> to just a u16, which is probably unbeatable. But it isn't clear whether returning Result<u16, ()> could really be faster than Result<u16, u16>.

If that optimization does matter, we can probably hide the new field behind a feature and have people want it to opt in.

[seanmonstar]

Thanks for the PR! I see now by the implementation you mean to preserve the u16 which is out-of-range in the InvalidStatusCode error type. Do you feel this provides substantial benefits over being able to just look at the original u16 before calling StatusCode::from_u16?

[upsuper]

After some discussion, I don't think this is going to work for the usecase. Closing now.

Sorry for the noise.