Ensure database is dropped before asserting it doesn't exist

Versions of MongoDB before 3.0 create a database when dropping the collection of a database that doesn't exist (as is done by the setUp() method).

tests/Operation/DropDatabaseFunctionalTest.php

@@ -27,6 +27,9 @@ class DropDatabaseFunctionalTest extends FunctionalTestCase
     {
         $server = $this->getPrimaryServer();
 
+        $operation = new DropDatabase($this->getDatabaseName());
+        $operation->execute($server);
+
         $this->assertDatabaseDoesNotExist($server, $this->getDatabaseName());
 
         $operation = new DropDatabase($this->getDatabaseName());