[9.x] Force countBy method in EloquentCollection to return base collection

[jnoordsij]

Currently, the countBy method returns a static instance. However, the returned type is always of the form Collection<array-key, int>, so is never an Eloquent collection (in 'logical' sense). This can cause issues when chained with other collection methods, e.g. when doing something like $foo->countBy('bar')->merge($baz);.

Note: an alternative (and perhaps more accurate?) fix might be to change static to self on the base collection, though this is possibly a breaking change on those extending this class.