JSON::Pure fix strict mode

[casperisfine]

Followup: #519
Fix: #584

[eregon]

I also noticed the docs in https://github.com/flori/json/blob/4f876a8bf7c45972800bae5bdbaf892aa2f714a5/lib/json/pure/generator.rb#L222 are wrong, could you fix that too?

And the alignment in https://github.com/flori/json/blob/4f876a8bf7c45972800bae5bdbaf892aa2f714a5/lib/json/pure/generator.rb#L143 is inconsistent (I dislike this kind of alignment, too high maintenance and diff noise, but might as well be consistent in one way, i.e. all or none aligned)

[casperisfine]

I also noticed

I'd rather not bundle multiple unrelated fixes in the same PR. Makes merging more complicated.

[eregon]

Ah sorry, it's @script_safe misaligned, and that's unrelated indeed.

OTOH I think fixing the description of def strict? is very related (but not necessary in this PR, fair enough).

[eregon]

Thanks for the fix!