[WFCORE-95] fix *ListAttributeDefinition.Builder copy constructor

do not call setElementValidator() from the copy constructor as it
invalidates the validator of the attribute definition (corresponding to
the listValidator)

This only affects Primitive and Simple subclasses.

JIRA: https://issues.jboss.org/browse/WFCORE-95
Merge pull request #170 from jmesnil/WFCORE-95_PrimitiveListAttributeDefinition.Builder_copy_ctor

[WFCORE-95] fix PrimitiveListAttributeDefinition.Builder copy constructor