Make MySQL return "number of found rows" instead of number of "affected rows".
Document why we have to check with defined() first.
Added test of OCS privatedata to trigger key duplication
Merge pull request #9672 from owncloud/mysql-affectedrows

Make MySQL affected row count consistent with other DBMSes

* owncloud/mysql-affectedrows:
  Added test of OCS privatedata to trigger key duplication
  Document why we have to check with defined() first.
  Make MySQL return "number of found rows" instead of number of "affected rows".