Added test for individual field updates described in commit 5e93b939e059bd296104fb6150a7f7f29f5eeba0

- Tested 'inc' with 'popularity' field
- Tested 'add' with multivalued 'word_ss' field (assuming add+commit functionality works)

Related Explanation:
http://wiki.apache.org/solr/UpdateXmlMessages#Optional_attributes_for_.22field.22