Revert "this always sets :public to false, so just do that"

This reverts commit cae2b5bb.

I am an idiot.

actionpack/lib/action_controller/metal/data_streaming.rb

@@ -169,7 +169,7 @@ def send_file_headers!(options)
         # after it displays the "open/save" dialog, which means that if you
         # hit "open" the file isn't there anymore when the application that
         # is called for handling the download is run, so let's workaround that
-        response.cache_control[:public] = false
+        response.cache_control[:public] ||= false
       end
   end
 end