Allow SGML attributes to accept objects which respond to to_h #2270

Job	Run time
rubocop	36s
tests (ubuntu-latest, 3.3)	35s
tests (macos-latest, 3.2)	28s
tests (ubuntu-latest, head)	1m 18s
tests (ubuntu-latest, truffleruby)	1m 38s
tests (macos-latest, truffleruby-head)	1m 36s
tests (macos-latest, 3.3)	30s
tests (ubuntu-latest, 4.0)	28s
tests (ubuntu-latest, 3.2)	32s
tests (macos-latest, head)	2m 12s
tests (macos-latest, 3.4)	41s
tests (ubuntu-latest, 3.4)	34s
tests (macos-latest, 4.0)	23s
tests (ubuntu-latest, truffleruby-head)	1m 55s
tests (macos-latest, truffleruby)	1m 49s
	15m 15s

Provide feedback