[ruby/reline] Prevent a warning for `warning: literal string will be

frozen in the future`
(ruby/reline#744)

ruby/reline@69c95c8b6a

test/reline/helper.rb

@@ -96,7 +96,7 @@ class Reline::TestCase < Test::Unit::TestCase
       end
     }.join
   rescue Encoding::UndefinedConversionError, Encoding::InvalidByteSequenceError
-    input.unicode_normalize!(:nfc)
+    input = input.unicode_normalize(:nfc)
     if normalized
       options[:undef] = :replace
       options[:replace] = '?'