Extract redirect location handling and clarify behaviour.

[ioquatix]

Fixes https://github.com/socketry/async-http/issues/171.

Types of Changes

• New feature.
• Maintenance.

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.

[ioquatix]

@korbin I'll probably rename this class. I'll leave a compatibility shim in for a while. The scope has expanded beyond it's original name.