_processWhereIn does not allow the use of named query parameters

When writing a query such as

$query = $query->where('entity.myValue = :value', array(':value'=>5));

you are unable to then

$query = $query->whereIn('entity.otherValue', array(':otherValues'=>array(1,2,3)));

Doctrine complains that you may not mix positional and named query parameters.

The attached patch fixes this by checking if the key of the passed in parameter is non numeric and if so setting the "value" of the parameter place holder to the value of the key.