createTicket respects alert/autorespond negatives

It was previously not possible to use the `alert` and `autorespond` flags in a negative way to prevent those two events, since the ternary operator would have used the `false` ternary value, which was `true`.  This corrects that by allowing the default of `true` to be used only when the `$data` array does not have the appropriate keys set.  Assigning `$data['source']` was altered to use the same formatting and caution against an undefined index error.
XML supports false alert/autorespond, using isset

- Allows the string "false" to be used in the XML payload as the documentation demonstrates, and have that be interpreted as a boolean false.
- Switching $alert/$autorespond to use isset(), and forcing those variables to be type-casted to booleans.
Merge pull request #1435 from stevepacker/patch-1

createTicket respects alert/autorespond negatives

Reviewed-By: Peter Rotich <peter@osticket.com>