fix lang() to match whole language subtags, not any prefix

[rootvector2]

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NodePointer.isLanguage and its DOMNodePointer/JDOMNodePointer overrides test the language with String.startsWith, so lang() matches any leading character prefix of the node's xml:lang (or the locale name) instead of a whole subtag. I noticed it reading the lang() predicate path: a node with xml:lang="fr" satisfies lang('f'), and a locale of en-US satisfies lang('e') and lang('en-'), all of which must be false per XPath 1.0 section 4.3, which matches only when the argument equals the tag or a subtag delimited by -. Since lang() is used in predicates to select nodes, the over-match changes which nodes a filter returns for caller-supplied xml:lang data. The fix requires an exact case-insensitive match or a lang- prefix, in one shared helper used by all three implementations. The added assertions fail on the current code (lang('f') and lang('e') return true) and pass with the change.