[8.x] Handle collection creation around a single enum

[tontonsb]

Current behaviour:

$c = collect(SomeEnum::myCase);
$c->items; // ['name' => 'myCase']

$c = collect(BackedEnum::myCase);
$c->items; // ['name' => 'myCase', 'value' => BackedEnum::myCase->value]

In my opinion it is a bug, it is natural to expect collect(SomeEnum::myCase) to produce a single item collection containing the item itself like it does with collect('mystring') instead of unwrapping it and changing the item count based on whether it's a backed enum.

This PR changes the behaviour so that:

$c = collect(SomeEnum::myCase);
$c->items; // [SomeEnum::myCase]