From the description: Update ```_parseKey()``` signature so that ```Model $Model = null``` can be set as last argument, like ```conditions()```, ```conditionKeysToString()```, ```order()``` and ```group()```.

Albeit being consistent with the other methods, it is convenient to move it to the last one because it is the only one that has a predefined ```null``` value, making it easier to just avoid it (like the other methods).