XMLDSig XPath Filter 2 Transform subtract filter is broken

See https://issues.apache.org/bugzilla/show_bug.cgi?id=38444 for more information. This is a bug in the Apache XML Security implementation that was fixed after we released JDK 6. We should also fix this bug in JDK 6, as it is serious and is blocking some users from using the XPath Filter 2 Transform in their signatures.