Use native downcase and upcase

If another library has overridden these methods to fold the case of
characters outside the ASCII range then the user will get back something
closer to what he or she expects.

lib/babosa/utf8/dumb_proxy.rb

@@ -14,14 +14,13 @@ module DumbProxy
       extend self
 
       def downcase(string)
-        string.unpack("U*").map {|char| Mappings::DOWNCASE[char] or char}.flatten.pack("U*")
+        string.downcase.unpack("U*").map {|char| Mappings::DOWNCASE[char] or char}.flatten.pack("U*")
       end
 
       def upcase(string)
-        string.unpack("U*").map {|char| Mappings::UPCASE[char] or char}.flatten.pack("U*")
+        string.upcase.unpack("U*").map {|char| Mappings::UPCASE[char] or char}.flatten.pack("U*")
       end
 
-
       if String.public_instance_methods.include?(:unicode_normalize)
         def normalize_utf8(string)
           string.unicode_normalize