Add test for overriding qualifier null default (#73)
Ignore qualifier defaults on non-initial desires (#73)

DAG updates incorrectly applied qualifier defaults to
non-initial desires due to an erroneous check for the
desire's non-initial status.  This checks the chain
length (correct), not desire equality (incorrect).
add missing license header
Merge pull request #74 from grouplens/feature/elehack-73-null-inject

Fix null injection (fixes #73)