So the problem is that xmlns is not inherited when building a stanza,
and then :get_child(n, ns) with an explicit namespace does not find that
such child tags.
E.g.
local t = st.stanza("foo", { xmlns = "urn:example:bar" })
:text_tag("hello", "world");
assert(t:get_child("hello", "urn:example:bar"), "This fails");
Meanwhile, during parsing (util.xmppstream or util.xml) child tags do
get the parents xmlns when not overriding them.
Thus, in the above example, if the stanza is passed trough
`t = util.xml.parse(tostring(t))` then the assert succeeds.
This change makes it so that it leaves out the namespace argument to
:get_child when it is the same as the current/parent namespace, which
behaves the same for both built and parsed stanzas.
