Use `jQuery.is(':disabled')` instead of `jQuery(':disbled').length` for Opera

Some tests are failed in Opera(12.15) - Mac OS X.

I fixed failing test by using `jQuery.is(':disable')` instead of `jQuery(':disable').length`.

In the case of `element.disabled = true;`, Opera has it as `<input  disabled="true"/>` (not `<input  disabled="disabled"/>`).
It is not match with `$(':disabled')` and returns always `[]`.