Ensure that we cache attachment's file name before we perform assignment

There was an issue on assignment another S3 attachment object to another object whereas calling #to_file on S3 attachment will create a Tempfile, which would then having a different file name than original version.

By caching the filename first before calling #to_file, we're ensuring that filename is the original one.

Fixes #9

lib/paperclip/attachment.rb

@@ -115,6 +115,7 @@ module Paperclip
       ensure_required_accessors!
 
       if uploaded_file.is_a?(Paperclip::Attachment)
+        uploaded_filename = uploaded_file.original_filename
         uploaded_file = uploaded_file.to_file(:original)
         close_uploaded_file = uploaded_file.respond_to?(:close)
       end
@@ -126,8 +127,9 @@ module Paperclip
 
       return nil if uploaded_file.nil?
 
+      uploaded_filename ||= uploaded_file.original_filename
       @queued_for_write[:original]   = to_tempfile(uploaded_file)
-      instance_write(:file_name,       uploaded_file.original_filename.strip)
+      instance_write(:file_name,       uploaded_filename.strip)
       instance_write(:content_type,    uploaded_file.content_type.to_s.strip)
       instance_write(:file_size,       uploaded_file.size.to_i)
       instance_write(:fingerprint,     generate_fingerprint(uploaded_file))