Do not enforce uniqueItems when it is set to false

[gaoflow]

Three-way control on [1, 1] against master: uniqueItems: true is correctly invalid, the keyword absent is correctly valid, and uniqueItems: false is wrongly invalid — same in draft2, draft3, draft4 and draft6. The cause is that Validator#validate dispatches an attribute whenever its keyword is present in the schema, and UniqueItemsAttribute.validate never reads current_schema.schema['uniqueItems'], so presence alone enforces the constraint; draft-04 §5.3.4.2 ("If this keyword has boolean value false, the instance validates successfully") and §5.3.4.3, which makes false the default, say it must be a no-op, as do draft-03 §5.15 and draft-06 §6.13. That matters in practice because an explicit false is the normal way to override an inherited uniqueItems: true, which is exactly the report in #461. The fix reads the value and returns early when it is falsy — the metaschema already constrains uniqueItems to a boolean, so false is the only value whose behaviour changes.

This survived because the official test suite does cover it — 36 cases across draft3/4/6, every one named uniqueItems=false … — but test/common_test_suite_test.rb builds its test methods from Dir[...] at class-definition time, and test/test-suite is empty in a plain checkout, so the file contributes nothing:

$ bundle exec ruby -Ilib -I. test/common_test_suite_test.rb
Finished in 0.04736s
0 tests, 0 assertions, 0 failures, 0 errors, 0 skips

.github/workflows/test.yml checks out without submodules:, and .gitmodules still points at git://github.com/…, which GitHub switched off in 2022 (git ls-remote on it hangs). I've left all of that alone since #387 is already open on it and it's your call — and no unit-test fixture had ever set uniqueItems to anything but true, so nothing else caught it either.

I ran bundle exec rake test (383 → 396 tests, 2359 → 2551 assertions, 0 failures) and bundle exec rubocop clean; reverting only the lib/ change while keeping the new tests fails 8 of them, and disabling the keyword outright fails the pre-existing test_unique_items in all four drafts. Cloning the pinned submodule commit 637f0ac locally and running draft3/4/6 through it takes failures from 161 to 125 with errors unchanged at 92 — exactly the 36 uniqueItems cases, and no other suite file's count moves. I also audited every other attribute class for the same "ignores its own keyword value" defect: all 38 keyword/class pairs registered across drafts 1–6, plus a true/false/absent differential over every boolean-valued keyword (additionalProperties, additionalItems, exclusiveMaximum/exclusiveMinimum, maximumCanEqual/minimumCanEqual, draft3 required, draft1/2 optional, draft6 propertyNames) — all of them read their value correctly, so uniqueItems is the only offender rather than a pattern needing a wider change.

The tests go into the shared ArrayValidation::UniqueItemsTests, which draft2/3/4 already include; I also included that module in Draft6Test, which had no uniqueItems coverage at all.

Closes #461.