Don't modify input string in place

lib/email_reply_parser.rb

@@ -62,7 +62,7 @@ def initialize
     def read(text)
       # The text is reversed initially due to the way we check for hidden
       # fragments.
-      text.reverse!
+      text = text.reverse
 
       # This determines if any 'visible' Fragment has been found.  Once any
       # visible Fragment is found, stop looking for hidden ones.

test/email_reply_parser_test.rb

@@ -71,6 +71,12 @@ def test_recognizes_date_string_above_quote
     assert_match /Loader/,   reply.fragments[1].to_s
   end
 
+  def test_does_not_modify_input_string
+    original = "The Quick Brown Fox Jumps Over The Lazy Dog"
+    EmailReplyParser.read original
+    assert_equal "The Quick Brown Fox Jumps Over The Lazy Dog", original
+  end
+
   def email(name)
     body = IO.read EMAIL_FIXTURE_PATH.join("#{name}.txt").to_s
     EmailReplyParser.read body