Fixed error that occurs when Cheerio context is a DOM element.

Hello,

I ran across an error that occurs when this bit of code is executed:

```javascript
var cheerio = require("cheerio");
var $ = cheerio.load("<b>hey <i>everybody</i>!</b>");
$("b").map(function() { return $("i", this).text(); })
// Exception thrown -- TypeError: Object #<Object> has no method 'find'
```

Which is unfortunately because I feel the example above is fairly idiomatic jQuery.  I took a look at the code (which is very well factored!) and put together a possible fix.

Thanks for taking a look.