remove errors Nokogiri errors

This code makes tries to do flatten operations on an array of
Nokogiri::XML::Builder::NodeBuilder objects created by Nokogiri
xml.send calls. These are superfluous given that the join call
creates a string which is promptly ignored and a
builder.to_stripped_xml() call is used to get the string returned.

Basically I it seems completely pointless and isn't the way Nokogiri
works. But I may be missing something.

This fixes a no-method: flatten error stopping Topics being saved.
Merge pull request #25 from kete/rr/nokogiri_fix

remove errors Nokogiri errors