Fix issue #32 by safely handling Events with closed tempfiles

This fixes [issue #32](https://github.com/dejan/rails_panel/issues/32) and lets MetaRequest co-exist happily with Dragonfly for me! It resolves the problem where trying to upload an image file was erroring out because AppRequestHandler was trying to json encode a MetaRequest::Event with a payload containing a closed tempfile.

Better to sidestep trying to read closed tempfiles here than to go into Dragonfly to make sure tempfiles stay open after images are uploaded and processed, since it's an issue that could potentially arise with other image processing libraries as well.