Additionally normalize to using $namespace rather than $uri for
namespace parameters, including in XMLReader and XMLWriter. I went
with that one as it is currently used by DOM, SimpleXML and XSL --
and our DOM parameter names follow the DOM specification.
Closes GH-6295.
Otherwise it is hard to uphold the arginfo contract for this
function -- we cannot simply mirror the zpp call, as the class
entry it is based on does not exist.
