(bug 34542) Calling mw.loader.load('http://someurlhere') sometimes calls document...

(bug 34542) Calling mw.loader.load('someurlhere') sometimes calls document.write() from inside an asynchronously loaded script. This is because we added the async parameter in 1.19, and made it default to false, which works for 1.18 HTML but doesn't work for user/site/Gadget JS written for 1.18 . So make the async parameter default to true when a URL is passed and to false otherwise. This is kind of evil but it's the only sane way of preserving b/c that I can think of