replace couchforms update handler with python equivalent
fix test for python xml2json in couchforms

insignificant test change:
test is tightly coupled to an exception message
which used to be an erlang dump
and is now a lxml sytax error message

specifically added the text Invalid XML to the error message
so I could check for that rather than something more tightly coupled
update couchforms ref
Merge branch 'replace-couchforms-update' into staging