fixes extraStatements bug for Spanner\Instance::createDatabase (#499)

* fixes extraStatements bug for Spanner\Instance::createDatabase

* fixes tests

Author: bshaffer

Connection/Grpc.php

@@ -297,7 +297,6 @@ public function createDatabase(array $args)
         $res = $this->send([$this->databaseAdminClient, 'createDatabase'], [
             $instanceName,
             $this->pluck('createStatement', $args),
-            $this->pluck('extraStatements', $args),
             $this->addResourcePrefixHeader($args, $instanceName)
         ]);