Sometimes you only care about a single attribute, but the child tag
itself may be optional, leading to needing `tag and tag.attr.foo` or
`stanza:find("tag@foo")`.
The `:find()` method is fairly complex, so avoiding it for this kind of
simpler use case is a win.
Assumes that most number fields are integers, as most numeric types
listed in XEP-0122 are, as are all such fields in Prosody as of this.
Otherwise %g produces something like 1.1259e+15
The stanza API does not accept number values and threw an error due to
the height and width attributes of the media element (XEP-0221).
This part had no test coverage previously, explaining why it was not
discovered until now.
This should allow the usage of long prefixes and namespace-like names to
be contained to the XML representation of the form, so that the code can
use more convenient names.
