Fix compatibility with --enable-frozen-string-literal

This has been a supported option of Ruby since Ruby 2.3, and
even without it, mutating a string literal emits a warning.

Author: byroot

.github/workflows/ruby.yml

@@ -22,6 +22,7 @@ jobs:
           - 3.2
           - 3.3
           - 3.4
+          - 4.0
           - head
 
     steps:
@@ -36,3 +37,6 @@ jobs:
       env:
         COVERALLS_REPO_TOKEN: ${{ secrets.COVERALLS_REPO_TOKEN }}
       run: bundle exec rake spec
+      env:
+        RUBYOPT: "--enable-frozen-string-literal --debug-frozen-string-literal"
+

lib/docx/containers/paragraph.rb

@@ -44,7 +44,7 @@ def to_s
 
         # Return paragraph as a <p></p> HTML fragment with formatting based on properties.
         def to_html
-          html = ''
+          html = +''
           text_runs.each do |text_run|
             html << text_run.to_html
           end