Unit Tests

- Corrected unit test that relied on old behavior, namely 'due.before:1d',
  which is like specifying '1403007839 < 86400'. Taskwarrior always assumed
  that whenever a duration was specified, such as '1d', it really meant
  'now + 1d'. With full algebraic expression support, this can no longer be
  the case and now we need 'due.before:now+1d'.
Unit Tests

- Corrected test to expect durations in ISO8601 format.