[#158] return a truthy value

- Per discussion with @Ocramius, a `null` value does not work currently
  until zendframework/zf2#6147 is in a released version. That patch
  fixes a `!$value` call to more correctly be a `false === $value` call;
  until then, a truthy, but not boolean true, return value will also
  still work.
Merge pull request #158 from weierophinney/hotfix/zf-apigility-56

#157 breaks compatibility with current Zend\Session