Bug 1101518 - [Build] Don't rewrite "async" scripts, and fix the skippin...

...g operation
I tried "script.async" and it was always true even for script without the "async" attribute. Moreoever, the "async" attribute is a bool attribute which means that `async="false"` is actually enabling it... see http://www.w3.org/TR/html5/scripting-1.html#attr-script-async and http://www.w3.org/TR/html5/infrastructure.html#boolean-attribute.

So `script.hasAttribute` is actually accurate, and this is what is actually used in Gecko, see:
* http://mxr.mozilla.org/mozilla-central/source/dom/html/HTMLScriptElement.cpp#183
* http://mxr.mozilla.org/mozilla-central/source/dom/base/Element.h#980

So I reverted back to this :)