[aws|storage] warn/load unf as needed

lib/fog/aws/storage.rb

@@ -174,6 +174,14 @@ module Fog
 
         # NOTE: differs from Fog::AWS.escape by NOT escaping `/`
         def escape(string)
+          unless @unf_loaded_or_warned
+            begin
+              require('unf/normalizer')
+            rescue LoadError
+              Fog::Logger.warning("Unable to load the 'unf' gem. Your AWS strings may not be properly encoded.")
+            end
+            @unf_loaded_or_warned = true
+          end
           string = defined?(::UNF::Normalizer) ? ::UNF::Normalizer.normalize(string, :nfc) : string
           string.gsub(/([^a-zA-Z0-9_.\-~\/]+)/) {
             "%" + $1.unpack("H2" * $1.bytesize).join("%").upcase