fix bug in Ono and add test

@mattt I was having problem with the CSS selector #foo where in my document I had 

```
<img id="foo" src="...">
````

I was getting:

```
XPath error : Invalid expression
//[@id = 'foo']
  ^
```

From http://ejohn.org/blog/xpath-css-selectors/

Looks like this is supposed to be:

```
//*[@id = 'foo']
```

And when I fix that, it works.  I've added a test for this, and all tests pass.

But I'm no XPath expert, so please let me know what you think.