Save reason phrase in Protocol::HTTP1::Response

[uberjay]

Save away the raw reason phrase provided by the server. This is of limited niche value, as the reason phrase seems to be mostly unused/ignored.

One example of where it's needed:

The Proxmox VE API ships error reason strings back using the HTTP reason phrase. Without this side-channel, error status is 100% opaque/unavailable.

See (very brief) discussion: https://github.com/socketry/async-http/discussions/145

It's probably not worth overthinking this, but some other thoughts that came to mind were:

• Does it make sense to define a reason accessor in other HTTP version responses so the method is always available?
  • My opinion: no, not worth it and feels pretty bad to burden other versions of HTTP with this detail when it's nearly irrelevant.
• Is it worth any sort of string optimization scheme? (e.g., String#dedup.)
  • My opinion: probably not, but happy to hear why it's a good idea.

Types of Changes

• New feature.

Contribution

• [x] I added tests for my changes.
• [x] I tested my changes locally.
• [x] I agree to the Developer's Certificate of Origin 1.1.