Merge pull request #82 from casperisfine/mri-27-frozen-string

Make Memoist.escape_punctuation compatible with MRI 2.7

The latest ruby-2.7 preview included some changes to `Symbol#to_s` so that it returns a frozen string [1].

This PR fixes an issue with the `Memoist.escape_punctuation` method so that we can maintain compatibility with this new behaviour potentially being introduced in Ruby 2.7.

[1]  https://bugs.ruby-lang.org/issues/16150

lib/memoist.rb

@@ -38,10 +38,12 @@ def self.unmemoized_prefix(identifier = nil)
   end
 
   def self.escape_punctuation(string)
-    string = string.is_a?(String) ? string.dup : string.to_s
+    string = string.to_s
 
     return string unless string.end_with?('?'.freeze, '!'.freeze)
 
+    string = string.dup if string.frozen?
+
     # A String can't end in both ? and !
     if string.sub!(/\?\Z/, '_query'.freeze)
     else