Fixed ajax calling by passing url string instead of settings object

Situation before:
when I called $.nette.ajax('http://example.com'), the internal jquery $.ajax('http://example.com') was processed successfully, however nette.ajax.js didn't wrapped it correctly (i.e. one of side effects was that 'before' event was not properly assigned and therefore not fired).

Fix:
if string is passed now, the settings object is converted: settings = {url: settings}, and now both nette.ajax.js and jquery is behaving proeprly.