[8.x] Add merge() function to ValidatedInput

[HajMo]

This PR will add the ability to merge items to validated inputs. As an example

    $validator = Validator::make(['name' => 'Taylor'], ['name' => 'required']);

    $validator->safe()->merge(['role' => 'Admin']);

    User::create($validator->safe()->all());

Previously you can't do this when using Form Request, you can't merge any data to the returned data from validated().

[driesvints]

I don't think this is wanted. The point of ValidatedInput is that it's data that's been validated and data you merge in may not be. I realize __set and offsetSet are defined on ValidatedInput as well but the later needs to be implemented to offer ArrayAccess support.

[garygreen]

I realize __set and offsetSet are defined on ValidatedInput as well but the later needs to be implemented to offer ArrayAccess support.

If this class is intended to be immutable then shouldn't the set method throw an exception rather than trying to set the value? That way it's still complies with ArrayAccess but keeps with the intended design goal.

[driesvints]

If this class is intended to be immutable then shouldn't the set method throw an exception rather than trying to set the value?

@garygreen I agree. I don't know though why it was added.

[taylorotwell]

I honestly don't mind this that much... sometimes you may need to throw a few default values into the validated data if you are going to pass it straight to a model creation method.

[driesvints]

No worries @taylorotwell. Was just wondering if you intended it to be mutable.

[driesvints]

Just a thought: would it be better if this method was used that a new non-ValidatedInput object was returned? Like a collection instance?

[taylorotwell]

@driesvints then Eloquent would no longer be able to use it for mass assignment.

[garygreen]

It's kind of a shame this class doesn't have a clearer design goal - it's essentially just a stripped back collection but called ValidatedInput instead. Maybe it could have just extended the collection instance? 🤷‍♀️