Update for frozen string compatibility #362

Job	Run time
Unit test (ubuntu-20.04, 2.1)	55s
Unit test (ubuntu-20.04, 2.2)	1m 6s
Unit test (ubuntu-20.04, 2.3)	1m 1s
Unit test (ubuntu-20.04, 2.4)	1m 2s
Unit test (ubuntu-20.04, 2.5)	27s
Unit test (ubuntu-20.04, 2.6)	28s
Unit test (ubuntu-20.04, 2.7)	29s
Unit test (ubuntu-20.04, 3)	29s
Unit test (ubuntu-20.04, 3.1)	22s
Unit test (ubuntu-20.04, 3.2)	15s
Unit test (ubuntu-20.04, 3.3)	16s
Unit test (ubuntu-20.04, jruby-9.1)	55s
Unit test (ubuntu-20.04, jruby-9.2)	58s
Unit test (ubuntu-20.04, jruby-9.3)	55s
Unit test (ubuntu-20.04, jruby-9.4)	51s
Unit test (ubuntu-20.04, truffleruby)	1m 53s
Unit test (macos-latest, 2.6)	37s
Unit test (macos-latest, 2.7)	42s
Unit test (macos-latest, 3)	20s
Unit test (macos-latest, 3.1)	24s
Unit test (macos-latest, 3.2)	19s
Unit test (macos-latest, 3.3)	19s
Unit test (macos-latest, jruby-9.3)	34s
Unit test (macos-latest, jruby-9.4)	44s
Unit test (macos-latest, truffleruby)	1m 16s
Unit test (windows-latest, 2.2)	1m 42s
Unit test (windows-latest, 2.3)	1m 29s
Unit test (windows-latest, 2.4)	1m 42s
Unit test (windows-latest, 2.5)	2m 36s
Unit test (windows-latest, 2.6)	3m 0s
Unit test (windows-latest, 2.7)	2m 43s
Unit test (windows-latest, 3)	2m 7s
Unit test (windows-latest, 3.1)	1m 18s
Unit test (windows-latest, 3.2)	1m 19s
Unit test (windows-latest, 3.3)	2m 3s
finish	0s
	37m 36s

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