Updated setParameters function for not replace all parameters

When you create QueryBuilder and you bind some parameters, if you use setParameters function after bind other parameters, previous parameters are removed(https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/QueryBuilder.php#L558). This would not be a problem if it was the same behaviour with the attribute $_parameterMappings (in ParserResult), but it's not the case, given that it keeps the number of parameters introduced, throwing an exception(https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/Query.php#L293). To prevent this, I've made it initialize the setParameters with the existing ones.