Update change in Set Mutator syntax

As per this commit and its discussion https://github.com/illuminate/database/commit/be7246d44f4667e27a196cbf91225f758b862004#L0R916 and then this issue, https://github.com/illuminate/database/issues/107 the syntax for mutators has changed.
Merge pull request #6 from Kindari/patch-1

Update change in Set Mutator syntax