Add frozen string literal to production code #396

Job	Run time
Ruby specs (2.6, rails_6.0, stable)	1m 53s
Ruby specs (2.6, rails_6.1, stable)	2m 22s
Ruby specs (2.7, rails_6.0, stable)	2m 0s
Ruby specs (2.7, rails_6.1, stable)	1m 57s
Ruby specs (2.7, rails_7.0, stable)	1m 54s
Ruby specs (3.0, rails_6.0, stable)	2m 0s
Ruby specs (3.0, rails_6.1, stable)	1m 42s
Ruby specs (3.0, rails_7.0, stable)	2m 3s
Ruby specs (3.1, rails_7.0, stable)	1m 46s
Ruby specs (3.2, rails_7.0, stable)	1m 47s
Ruby specs (2.7, rails_edge, experimental)	2m 31s
Ruby specs (3.0, rails_edge, experimental)	1m 47s
Ruby specs (3.1, rails_edge, experimental)	2m 10s
Ruby specs (3.2, rails_edge, experimental)	1m 51s
Ruby specs (head, rails_7.0, experimental)	1m 56s
Ruby specs (head, rails_edge, experimental)	2m 15s
	31m 54s

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