CRM-16036 - Fix for chaining.

CRM-16036 - Fix for chaining.

When using a custom field as param for the API, chaining
didn't work as it should. This is a fix.

I have a unit test for this as well, but it depends on a fix for
CRM-16168, so if I add it here, it will break anyway.

I will document this in the issue itself.

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* CRM-16036: API: searching on custom fields does not work
  https://issues.civicrm.org/jira/browse/CRM-16036
* CRM-16168: Chained call event-loc block returns an error if no loc block exists.
  https://issues.civicrm.org/jira/browse/CRM-16168