Add frozen string literal to production code #394

Job	Run time
Ruby specs (2.6, rails_6.0, stable)	2m 1s
Ruby specs (2.6, rails_6.1, stable)	1m 55s
Ruby specs (2.7, rails_6.0, stable)	1m 55s
Ruby specs (2.7, rails_6.1, stable)	1m 58s
Ruby specs (2.7, rails_7.0, stable)	2m 0s
Ruby specs (3.0, rails_6.0, stable)	1m 40s
Ruby specs (3.0, rails_6.1, stable)	1m 39s
Ruby specs (3.0, rails_7.0, stable)	1m 49s
Ruby specs (3.1, rails_7.0, stable)	2m 1s
Ruby specs (3.2, rails_7.0, stable)	1m 58s
Ruby specs (2.7, rails_edge, experimental)	2m 1s
Ruby specs (3.0, rails_edge, experimental)	1m 48s
Ruby specs (3.1, rails_edge, experimental)	1m 47s
Ruby specs (3.2, rails_edge, experimental)	1m 45s
Ruby specs (head, rails_7.0, experimental)	1m 58s
Ruby specs (head, rails_edge, experimental)	2m 0s
	30m 15s

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