Allow SGML attributes to accept objects which respond to to_h #2207

Job	Run time
tests (ubuntu-latest, 3.4)	33s
tests (macos-latest, 3.2)	44s
tests (macos-latest, 3.4)	23s
tests (ubuntu-latest, 3.2)	37s
tests (macos-latest, head)	1m 12s
tests (ubuntu-latest, truffleruby-head)	1m 34s
tests (macos-latest, truffleruby)	1m 3s
tests (ubuntu-latest, truffleruby)	1m 33s
tests (macos-latest, truffleruby-head)	1m 11s
tests (ubuntu-latest, head)	1m 13s
tests (ubuntu-latest, 3.3)	30s
tests (macos-latest, 3.3)	30s
rubocop	42s
	11m 45s

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