Fix to handle hex numeric XML entities

If the parser encounters a hex entity, eg `&#xA;`, `entitySwap` correctly
identifies it as a numeric entity but causes a 'bad argument' error as
Lua requires hex numbers to be prefixed with "0x", i.e. 0xA not xA

By prepending "0" to the numeric portion of the entity, hex values are
correctly formed and decimal values are still valid, e.g. 0xA & 010