SumAccumulator should behave according to JPA spec

Currently it just preserves the type of the field, which can easily lead to overflow. The JPA spec tries to avoid it by widening the numeric type, see 10.2.7.4. JPQL Aggregate Functions of JPA spec here: http://docs.oracle.com/cd/E12839_01/apirefs.1111/e13946/ejb3_langref.html