add LIMIT on update query

[safwanghamrawi]

When we make an update query builder with setMaxResults to add LIMIT, the LIMIT is ignored in the class SqlWalker method walkUpdateStatement. Could we modify this function to be like: `

/**
 * {@inheritdoc}
 */
public function walkUpdateStatement(AST\UpdateStatement $AST)
{
    $this->useSqlTableAliases = false;
    $this->rsm->isSelect      = false;
    $limit    = $this->query->getMaxResults();

    $sql = $this->walkUpdateClause($AST->updateClause)
        . $this->walkWhereClause($AST->whereClause);

    if ($limit !== null) {
        $sql = $this->platform->modifyLimitQuery($sql, $limit, null);
    }
    return $sql;
}

[Ocramius]

@safwanghamrawi a test case showing a failure caused by this is required before considering any fixes.