Correct implementation of `$.fn.text`

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