Fixed 'Can't set headers after they are sent.' exception on /keystone/[list]/?new

When clicking the 'new something' button on the item view, `renderView` is called twice inside `list.js` and causing express to throw 'Can't set headers after they are sent.' exception. This crashes the server.

This pull fixes it, but one issue still remains. If the model you are creating have required fields, an exception is thrown from mongoose. This does not crash the server, but no model is created.

Perhaps the button should be removed, and save takes you back to the list view?