Export should always be done with upsert

Attached patch replaces the if/else create/update workaround with Salesforce's idempotent upsert method.

From the documentation:

/**
* Creates new objects and updates existing objects; uses a custom field to
* determine the presence of existing objects. In most cases, we recommend
* that you use upsert instead of create because upsert is idempotent.
* Available in the API version 7.0 and later.
*
* @param string $ext_Id External Id
* @param array $sObjects Array of sObjects
* @return UpsertResult
*/
public function upsert($ext_Id, $sObjects)