Handle case where server responds with 204 No Content

If the server responds with 204 No Content, then jquery passes an
object to the success handler, instead of the string you'd expect.
We therefore need to check that what we're asked to parse is indeed
a string.
Use prop() ito attr() to deselect radio buttons

There is no need to go via atributes for things that are proper
boolean properties. Also, setting the attribute to an empty
string didn't work in firefox.
Only do list marshalling for multiple selection
Merge pull request #3 from izak/master

Handle case where server responds with 204 No Content, use jQuery.prop() ito attr() to deselect radio buttons, this works for radio buttons on on firefox, don't use :list marshalling for single select.