Make the test suite compatible with --enable-frozen-string-literal

[casperisfine]

Ref: https://bugs.ruby-lang.org/issues/20205

Since rexml is tested as part of ruby-core CI, it needs to be compatible with the --enable-frozen-string-literal option.

[kou]

Could you also add a CI job with --enable-frozen-string-literal?

diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 94a116a..af042af 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -33,6 +33,18 @@ jobs:
       - name: Test
         run: bundle exec rake test
 
+  frozen-string-literal:
+    name: frozen-string-literal
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ruby
+          bundler-cache: true
+      - name: Test
+        run: bundle exec ruby --frozen-string-literal -S rake test
+
   gem:
     name: "Gem: ${{ matrix.ruby-version }} on ${{ matrix.runs-on }}"
     runs-on: ${{ matrix.runs-on }}

[casperisfine]

Done.

[kou]

Thanks.

[casperisfine]

Thanks for the merge ❤️