Don't assume the response has a content-type header

This will happen when the connection is reset, for example.
Fixed bug with FileUpload and empty values.
Add a loading cursor while ajax is in flight

Fixes #208
Render the working_copy if there is no published node

Not really correct, but what other choice do we have?  Normally this
will only happen when an admin is previewing an unpublished page, so
it's ok to show them the working copy.
Merge pull request #217 from fusionbox/empty_file_field_error

Fixed bug with FileUpload and empty values.