Correct implementation of `$.fn.text`

This should resolve issue #255

Commit message:

> This method should only act as a "getter" when the first argument is
> undefined. If any other "falsey" value is specified, it should be
> coerced to its String form and set as the node's text content.
> 
> The analogous logic in jQuery passes the value to the DOM
> `createTextNode` method, which behaves in this way:
> 
> https://github.com/jquery/jquery/blob/2.0.3/src/manipulation.js#L35-L37