[ActiveRecord] Deduplicate optimizer hints

[kirs]

When you merge two relations both having the same optimizer hint (it could come from a default_scope, for instance), you'll end up with duplicated hints in the result query.

This patch fixed that by applying uniq to hints.

cc @kamipo @rafaelfranca

[kirs]

@kamipo @kaspth thanks for feedback, updated!

[kamipo]

This is not expected query for postgresql and sqlite3 adapters.

https://buildkite.com/rails/rails/builds/62311#86409ae0-8a89-4be7-8d6b-e7b6757648da/998-1009
https://buildkite.com/rails/rails/builds/62311#888c2216-dfbf-490a-8123-eed0a3aa1912/985-994

[kamipo]

Existing similar test case would be of your help.

rails/activerecord/test/cases/annotate_test.rb

Lines 9 to 16 in d3b951f

	def test_annotate_wraps_content_in_an_inline_comment
	quoted_posts_id, quoted_posts = regexp_escape_table_name("posts.id"), regexp_escape_table_name("posts")
	assert_sql(%r{\ASELECT #{quoted_posts_id} FROM #{quoted_posts} /\* foo \*/}i) do
	posts = Post.select(:id).annotate("foo")
	assert posts.first
	end
	end

[kirs]

Thanks! Fixed.