Associate an object to a key on this node. The object can later be retrieved from this node by calling getUserData with the same key.

Parameters:

key     The key to associate the object to.

data     The object to associate to the given key, or null to remove any existing association to that key.

handler     The handler to associate to that key, or null.

Returns: Returns the DOMUserData previously associated to the given key on this node, or null if there was none.

Since: 1.5, DOM Level 3

`key`		The key to associate the object to.
`data`		The object to associate to the given key, or `null` to remove any existing association to that key.
`handler`		The handler to associate to that key, or `null`.