Update remaining id774.net test URLs to HTTPS

[id774]

Summary

• The prior HTTPS migration of README, LICENSE, and source header id774.net links has already been merged into master.
• This PR mechanically updates the remaining id774.net URLs found in test fixtures and expected values (9 files) from http://id774.net to https://id774.net.
• Both input and expected values were updated together wherever the same URL appeared in both, so no test's actual result changed.
• HTTP fixtures pointing to hosts other than id774.net (e.g. example.com, github.com, matome.naver.jp), and tests that deliberately exercise HTTP/HTTPS scheme-matching behavior (e.g. "when the link is https and the record is anchored on http"), were intentionally left unchanged.
• No changes to production code, version, documentation, or dependencies.

Changed files

• spec/plugins/filter/image_spec.rb
• spec/plugins/store/permalink_spec.rb
• spec/plugins/subscription/xml_spec.rb
• spec/plugins/subscription/link_spec.rb
• spec/plugins/filter/absolute_uri_spec.rb
• spec/plugins/filter/full_feed_spec.rb
• spec/plugins/provide/fluentd_spec.rb
• spec/plugins/store/file_spec.rb
• test/integration/test_xml2fluentd.yml

Validation (actual results)

• Remaining old http://id774.net URLs in the 9 target files: 0 — PASS
• Changes outside the 9 target files: 0 — PASS
• Non-id774.net HTTP URLs changed: 0 — PASS
• HTTP/HTTPS scheme-matching test semantics unchanged: PASS
• Production code changes: 0 — PASS
• Version changes: 0 — PASS
• Documentation changes: 0 — PASS
• Dependency changes: 0 — PASS
• git diff --check: PASS (0 whitespace errors)
• bundle exec rake: PASS (423 examples, 0 failures; pre-existing optional-plugin specs were skipped per existing policy due to missing optional gems, unrelated to this change)

🤖 Generated with Claude Code