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Fix yard warnings #521

Job	Run time
other (ubuntu, truffleruby)	2m 37s
valgrind	4m 25s
other (ubuntu, jruby-9.4)	39s
head-versions (ubuntu, ruby-head)	1m 8s
head-versions (ubuntu, jruby-head)	42s
mri (ubuntu-latest, 4.0)	29s
mri (ubuntu-latest, 3.4)	33s
mri (ubuntu-latest, 3.3)	28s
mri (ubuntu-latest, 3.2)	28s
mri (ubuntu-latest, 2.7)	36s
mri (macos-latest, 2.6)	40s
mri (ubuntu-latest, 3.5)	24s
mri (macos-latest, 3.2)	25s
mri (ubuntu-latest, 3.1)	26s
mri (ubuntu-latest, 3.0)	27s
mri (ubuntu-latest, 2.5)	28s
mri (windows-latest, 3.2)	1m 47s
mri (windows-latest, 2.5)	2m 14s
mri (ubuntu-latest, 2.6)	35s
	19m 31s