r88077 [1] added a default XML namespace (xmlns attribute on the root node)
for the API's XML output.
In r99135 [2] this was partially backed out to avoid clobbering some
modules that specified their own XML namespaces in output, such as RSD
discovery info. With this change a query parameter needed to be added to
include the namespace, but it would still clobber if you used it on
affected modules.
In r101912 [3] I took the parameter out, returning almost to r88077's
behavior -- but swapping the order of things so an xmlns contained in the
output data will take precedence, avoiding the clobbering.
However I do want to confirm that this is an OK thing to do; as noted in a
CR comment on [1] this may actually break XPath queries assuming the
non-namespaced data. If there is a compat issue, we need to make sure it's
documented and communicated...
[1] <https://www.mediawiki.org/wiki/Special:Code/MediaWiki/88007>
[2] <https://www.mediawiki.org/wiki/Special:Code/MediaWiki/99135>
[3] <https://www.mediawiki.org/wiki/Special:Code/MediaWiki/101912>
-- brion