Fix Columnar sort priority crash after mutation

[GabrielNagy]

When a setter like extensions= clears @__sort_priority and the container is not fully loaded, Columnar#update_sort_priority reads bits from nil instead of recomputing. Fall back to super in that case.

Short repro script:

MIME::Types["text/plain"].first.add_extensions("json")
MIME::Types.type_for("test.json")

~/.gem/ruby/3.3.6/gems/mime-types-3.7.0/lib/mime/types.rb:155:in `sort': comparison of MIME::Type::Columnar with MIME::Type::Columnar failed (ArgumentError)

[coveralls]

coverage: 88.743% (-0.3%) from 89.035%
when pulling 31a39f6 on GabrielNagy:fix-columnar-sort-priority-after-clear
into 7de6c8d on mime-types:main.