Parse JPA <exclude-unlisted-classes> correctly

Fix PersistenceUnitReader to correctly read <exclude-unlisted-classes>
in both JPA 1.0 and 2.0 persistence.xml files.

Prior to this commit PersistenceUnitReader would set the value of
excludeUnlistedClasses to true when a <exclude-unlisted-classes> element
was present, regardless of its value.

The following rules are now used when parsing:

- If the <exclude-unlisted-classes> element is missing the appropriate
  default value is set (based on the JPA version).

- If an empty <exclude-unlisted-classes/> element is found the
  excludeUnlistedClasses property is set to true.

- Otherwise the value of the <exclude-unlisted-classes> element is used
  to set the excludeUnlistedClasses property.

Issue: SPR-10767
Merge pull request #318 from beamerblvd/SPR-10767

# By Nick Williams
* SPR-10767:
  Parse JPA <exclude-unlisted-classes> correctly