[7.x] Adds higher order "when" proxy

[kayrunm]

As per the suggestion in laravel/ideas#1881, this PR adds the ability to use higher order messaging when using the when (and unless) functions in query builders/collections.

This would make using the when (and unless) functions a bit cleaner when someone is only adding one conditional method call, e.g.:

// Before...
User::query()
    ->when($condition, function ($query) use ($bar) {
        $query->where('foo', $bar);
    })
    ->when($another, function ($query) use ($qux) {
        $query->where('baz', $quz)
    })
    ->get();

// After...
User::query()
    ->when($condition)->where('foo', $bar)
    ->when($another)->where('baz', $qux)
    ->get();

[kayrunm]

The two failing tests in the first build (for PHP 7.2) don't seem to be related to these changes, as far as I can tell, as per #29777

[n10000k]

#29777

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If possible, please consider releasing your code as a package so that the community can still take advantage of your contributions!

If you feel absolutely certain that this code corrects a bug in the framework, please "@" mention me in a follow-up comment with further explanation so that GitHub will send me a notification of your response.

[Arkanius]

That's sad, really sad. @kayrunm could you please consider to create a package with this?

[kayrunm]

That's sad, really sad. @kayrunm could you please consider to create a package with this?

Given the very limited scope of the package I don't really see the point in creating one given some of the hoops that would have to be jumped through. Sorry, @Arkanius!

The code's there for you to see if you want to do it yourself, though.