setAttribute mutator can now return value to be set on the attribute

The purpose of the `set mutator` is to change the value before setting it to the attribute. It would be easier to do so if you could just return the new value from the `set...Attribute` method.

Right now you have to write `$this->attributes[$key] = your_processing($value);`

The `return $this->{$method}($value);` makes not much sense, as the only thing you would most likely want to return from setter is `$this`, which can be done later at the end of `setAttribute` method.