GROOVY-6621 fix Node#text when value is NodeList

by accounting for non-String children elements. Also added  short-cut case for NodeList value.

When groovy.util.Node#text checks for it's value being an instance of Collection, it only accounts for String children elements, which means the text() values of Node children of a NodeList value will not be part of the result.

Before this change, the testTextEmpty and testTextBasic test cases would pass, but the testTextWithChildren test could fail.

[Jira issue](http://jira.codehaus.org/browse/GROOVY-6621)