Fix jQuery #14142: Correctly handle numeric ids on Safari

cc @timmywil

It means more comparisons, but at least they're strict. And the OP is right in [pointing out](http://bugs.jquery.com/ticket/14142) that [HTML5 allows numeric ids](http://www.whatwg.org/specs/web-apps/current-work/multipage/elements.html#the-id-attribute).

Compared to master @ e1fc2f891b40419c52beb2ac2817a77399079dfe
```
   raw     gz 
  +157    +64 dist/sizzle.js
   +18    +10 dist/sizzle.min.js
```