keyIds should not contain null-entries

- this is especially relevant when new models with null-ids are matched
against keyIds and a false-match (null === null) is found
added test-case for supplied pull-request (keyIds should not contain
null-entries)
ignoring 'falsy' keyContents except 0
adding only non-falsy itemIds to keyIds array
Merge pull request #277 from ClemensSchneider/keyIds_should_not_contain_null_entries

keyIds should not contain null-entries