Fix JSON::Fragment#to_json signature #777

Job	Run time
Ruby memcheck	1m 8s
ruby-versions / Generate Ruby versions	3s
ubuntu-latest 2.7	1m 3s
ubuntu-latest 3.0	33s
ubuntu-latest 3.1	1m 54s
ubuntu-latest 3.2	34s
ubuntu-latest 3.3	39s
ubuntu-latest 3.4	2m 2s
ubuntu-latest head	1m 28s
ubuntu-latest jruby	1m 5s
ubuntu-latest jruby-head	1m 9s
macos-14 2.7	1m 3s
macos-14 3.0	23s
macos-14 3.1	31s
macos-14 3.2	24s
macos-14 3.3	25s
macos-14 3.4	22s
macos-14 head	17s
macos-14 jruby	44s
macos-14 jruby-head	43s
windows-latest 2.7	3m 40s
windows-latest 3.0	1m 59s
windows-latest 3.1	2m 49s
windows-latest 3.2	2m 26s
windows-latest 3.3	2m 54s
windows-latest 3.4	2m 44s
windows-latest head	1m 20s
ubuntu-24.04-arm 3.4	46s
macos-13 3.4	52s
windows-latest mswin	1m 11s
ubuntu-latest jruby-9.4	1m 14s
macos-latest truffleruby-head	47s
ubuntu-latest truffleruby-head	2m 4s
	41m 16s

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