Add AbstractAdapter#inspect to fix encoding error

According to the discussion in #923, it seems like this should be the
only way to fix this encoding debug issue. I'm also going to coordinate
with aws-sdk to get this fix in the upstream.

Fixes #923, #301

lib/paperclip/io_adapters/abstract_adapter.rb

@@ -13,6 +13,10 @@ module Paperclip
       @tempfile.read(length, buffer)
     end
 
+    def inspect
+      "#{self.class}: #{self.original_filename}"
+    end
+
     private
 
     def destination