`Query.has_results` calls `.exists()` with wrong argument

[Flavio Curella]

The has_results method of the Query class (in django/db/models/sql/query.py), passes the using argument to the exists() method:

    def has_results(self, using):
        q = self.exists(using)
        compiler = q.get_compiler(using=using)
        return compiler.has_results()

but the signature of the exists method does not accept an argument to select the db connection. It only accepts an argument to limit the rows it should fetch:

    def exists(self, limit=True):
        # ... snip ...