fix for frozen strings

lib/rainbow.rb

@@ -86,7 +86,7 @@ def hide
     def wrap_with_code(code) #:nodoc:
       return self unless Sickill::Rainbow.enabled
 
-      var = self.clone
+      var = self.dup
       matched = var.match(/^(\e\[([\d;]+)m)*/)
       var.insert(matched.end(0), "\e[#{code}m")
       var.concat("\e[0m") unless var =~ /\e\[0m$/

test/rainbow_test.rb

@@ -112,6 +112,12 @@ def test_immutability
     assert_equal string, "hello"
   end
 
+  def test_frozen
+    string = "frozen"
+    string.freeze
+    string.color(:red)
+  end
+
   class MyString < String
   end