Recognize XML declarations when detecting response encoding

[OskarEichler]

Summary

Recognize the <?xml declaration prefix when detecting response encoding. The existing expression matches <xml instead, missing real declarations and interpreting an ordinary xml element as one.

Reproduction

An XML response with body_encoding = true and a binary body beginning <?xml version="1.0" encoding="ISO-8859-1"?> is tagged UTF-8 despite the declaration. After the one-character correction, the response uses ISO-8859-1 without changing its bytes. Conversely, an ordinary <xml version="1.0" encoding="ISO-8859-1"/> element no longer changes the encoding.

The XML declaration grammar uses the <?xml prefix.

Verification

• Ruby 4.0.6 through rbenv; existing bundle exec rake test: 201 tests / 914 assertions / zero failures or errors, baseline and isolated patch.
• 32 focused checks cover four XML media types, single/double quotes, space/tab/newline separators, unchanged bytes, missing declarations, ordinary elements and explicit/header encoding precedence.
• Syntax and git diff --check pass; supplemental Lint scan has the same 25 baseline findings.
• No test/spec files were added or modified, per this contribution's no-new-tests constraint; focused reproductions ran externally.

Compatibility and limitations

No API/dependency/Ruby-requirement change. Intentional behavior correction: real declarations are recognized; ordinary xml elements are not treated as declarations. This is not a full XML parser and does not change media-type matching or the rest of the declaration grammar. Other Ruby/OS combinations and optional external OpenSSL checks were not run.