[5.6] Extract setting mutated attribute into method

[mnabialek]

When dealing with accessors, code looks like now like this:

if ($this->hasGetMutator($key)) {
    return $this->mutateAttribute($key, $value);
}

so in case of custom method functionality, it's possible to override both hasGetMutator and mutateAttribute for custom code.

But for mutator code looked so far like this:

if ($this->hasSetMutator($key)) {
    $method = 'set'.Str::studly($key).'Attribute';

    return $this->{$method}($value);
}

so it's impossible to do similar change for mutator. In fact you need to override the whole setAttribute method just to make similar change because setting mutated attribute is not extracted to external method.

After this PR code for mutator condition would look like this:

if ($this->hasSetMutator($key)) {
    return $this->setMutatedAttributeValue($key, $value);
}

so now it would be enough just to override those 2 methods instead of touching setAttribute code. Also code is more consistent now I believe.