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.

- 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.