Fix #607 exists() bug and assure compatibility with Laravel 5.1.19

[alexandre-butynski]

Laravel 5.1.19 version breaks the exists() method (see #607) because of a complete rewriting of the Laravel base method.

I think that the method should be rewrite in this package. This implementation is close to the best in term of performance but a small improvement can be done by using db.collection.find().limit(1)->count(1) but I don't know how to use the count method like that (if there is no argument, count() don't care about limit()).

If this pull request is accepted, a new release would be appreciated :)

[jenssegers]

Can't we use the same logic that Laravel had before instead of calling first()? Probably more efficient to force limit(1) on the exists query.

[alexandre-butynski]

$this->first() is faster than $this->limit(1)->count() because count() method ignores the effect of limit() method. A quick test on a 2 000 documents dataset shows a request 6 times faster.

See this doc page for more informations : http://docs.mongodb.org/manual/reference/method/cursor.count. The best performance would be reached by setting applySkipLimit to true to the cursor count() method but a simple find() is at least better than the previous solution.

[jenssegers]

Ok I will merge this until we find actual complications :)

[alexandre-butynski]

Thanks.

Maybe did I have made this PR in the wrong branch but this should be integrated in the 2.2 version of the package too !