Validation of xml:lang namespace fails
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The function DOMDocument__isValidNamespace insists on the binding of the "xml" prefix to the namespace "http://www.w3.org/XML/1998/namespace", while the specification states that this prefix doesn't have to be bound explicitly (http://www.w3.org/TR/2009/REC-xml-names-20091208/#xmlReserved). Therefore, the function should consider the empty string a valid namespace for the "xml" prefix.
Patched code