[extensions] Use StanzaView as parameter type in BoBDataExtension.from()
BoBDataExtensions can also appear in IQs (Registration IQ), and
potentially also Presence stanzas.
[core] Fix XmlEnvironment namespace: Use default namespace (not element's)
Assume the element
<foo:bar xmlns='namespace' xmlns:foo='foo-namespace'/>
then the <bar/> element's namespace is 'foo-namespace', but the
default namespace is 'namespace'. And this is the namespace that
scopes into inner elements.
