`String#strip_heredoc` preserves frozenness

```ruby
"foo".freeze.strip_heredoc.frozen?  # => true
```

Fixes the case where frozen string literals would inadvertently become
unfrozen:

```ruby

foo = <<-MSG.strip_heredoc
  la la la
MSG

foo.frozen?  # => false !??
```

1 files changed, 3 insertions, 1 deletions

diff --git a/activesupport/lib/active_support/core_ext/string/strip.rb b/activesupport/lib/active_support/core_ext/string/strip.rb
index cc26274e4a..6f9834bb16 100644
--- a/activesupport/lib/active_support/core_ext/string/strip.rb
+++ b/activesupport/lib/active_support/core_ext/string/strip.rb
@@ -20,6 +20,8 @@ class String
   # Technically, it looks for the least indented non-empty line
   # in the whole string, and removes that amount of leading whitespace.
   def strip_heredoc
-    gsub(/^#{scan(/^[ \t]*(?=\S)/).min}/, "".freeze)
+    gsub(/^#{scan(/^[ \t]*(?=\S)/).min}/, "".freeze).tap do |stripped|
+      stripped.freeze if frozen?
+    end
   end
 end