Bug 881 BeanMembersShouldSerialize - Skipping the setter check for final fields

[Bug 881](https://sourceforge.net/p/pmd/bugs/881/) has the [BeanMembersShouldSerialize](http://pmd.sourceforge.net/pmd-5.0.5/rules/java/javabeans.html#BeanMembersShouldSerialize) rule being applied to final fields -- but since they are final, they can't have setters.  This modification tweaks the rule to only check for a setter for fields that are not marked final.