When building the UPDATE query to send to the remote server, the
 federated storage engine built the query incorrectly if it was updating
 a field to be NULL.

 Thanks to Bjšrn Steinbrink for an initial patch for the problem.


mysql-test/r/federated.result:
  Add new results
mysql-test/t/federated.test:
  Add new regression test
sql/ha_federated.cc:
  Fix logic of how fields are added to SET and WHERE clauses of an
  UPDATE statement. Fields that were NULL were being handled incorrectly.
  Also reorganizes the code a little bit so the update of the two
  clauses is consistent.