Update for frozen string compatibility #363

Job	Run time
Unit test (ubuntu-20.04, 2.1)	54s
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)	56s
Unit test (ubuntu-20.04, 2.5)	27s
Unit test (ubuntu-20.04, 2.6)	28s
Unit test (ubuntu-20.04, 2.7)	28s
Unit test (ubuntu-20.04, 3)	15s
Unit test (ubuntu-20.04, 3.1)	17s
Unit test (ubuntu-20.04, 3.2)	20s
Unit test (ubuntu-20.04, 3.3)	19s
Unit test (ubuntu-20.04, jruby-9.1)	1m 7s
Unit test (ubuntu-20.04, jruby-9.2)	55s
Unit test (ubuntu-20.04, jruby-9.3)	46s
Unit test (ubuntu-20.04, jruby-9.4)	46s
Unit test (ubuntu-20.04, truffleruby)	1m 54s
Unit test (macos-latest, 2.6)	41s
Unit test (macos-latest, 2.7)	33s
Unit test (macos-latest, 3)	24s
Unit test (macos-latest, 3.1)	22s
Unit test (macos-latest, 3.2)	19s
Unit test (macos-latest, 3.3)	20s
Unit test (macos-latest, jruby-9.3)	36s
Unit test (macos-latest, jruby-9.4)	36s
Unit test (macos-latest, truffleruby)	1m 17s
Unit test (windows-latest, 2.2)	1m 44s
Unit test (windows-latest, 2.3)	1m 35s
Unit test (windows-latest, 2.4)	1m 34s
Unit test (windows-latest, 2.5)	2m 4s
Unit test (windows-latest, 2.6)	3m 6s
Unit test (windows-latest, 2.7)	3m 1s
Unit test (windows-latest, 3)	3m 40s
Unit test (windows-latest, 3.1)	1m 16s
Unit test (windows-latest, 3.2)	2m 4s
Unit test (windows-latest, 3.3)	3m 28s
finish	7s
	40m 46s

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