encoding/xml: attr does not work with pointer receiver.

<pre>See <a href="http://play.golang.org/p/OeSXsazIDS">http://play.golang.org/p/OeSXsazIDS</a>

If you unmarshal &lt;Param2 int=&quot;1&quot; /&gt;

into

type Param2 struct {
    Int *int `xml:&quot;int,attr&quot;`
}

Int will always be nil. As seen in the example, this works if Int is an element and not
an attribute.</pre>
