Additionally normalize to using $namespace rather than $uri for
namespace parameters, including in XMLReader and XMLWriter. I went
with that one as it is currently used by DOM, SimpleXML and XSL --
and our DOM parameter names follow the DOM specification.
Closes GH-6295.
The `$prefix` parameter of `xmlwriter_write_element_ns()` and
`xmlwriter_start_element_ns()` is nullable, what allows these functions
to be used instead of their non NS variants. Consequently, we make the
`$prefix` parameter of `xmlwriter_write_attribute_ns()` and
`xmlwriter_start_attribute_ns()` nullable as well.
