EUCA-7664
The jQuery fileupload plugin used by the wizards for uploading user data
does an iframe transport for non-XHR browsers like IE, but requires a response
content type of text/html, or IE intercepts the server response and offers to
open it in another application:

https://github.com/blueimp/jQuery-File-Upload/wiki/Frequently-Asked-Questions#why-does-internet-explorer-prompt-to-download-a-file-after-the-upload-completes

So, look at the request's Accept header and see if it can handle application/json or not,
and respond appropriately.
EUCA-7664
Doh! Set application/json by default.
EUCA-7664
Delete empty line, fix whitespace indents.
Merge pull request #98 from eucalyptus/EUCA-7664

Euca 7664 - look at the request's Accept header and see if it can handle application/json or not, and respond appropriately.