fix XMLSource issue in mappings wiki.
Mappings wiki MW is very old and does not include the <ns> element in it's schema.
Issue reported by [1] and was caused by #233 (from @nilesh-c)

[1] http://www.mail-archive.com/dbpedia-discussion@lists.sourceforge.net/msg06248.html
check if ns exists and require to be the same with wikiTitle
(fix according to comments)
Merge pull request #243 from jimkont/xmlsource

fix XMLSource issue in mappings wiki