Added bulk upload test for RM issue #2556.
Bulk Upload: Partial implementation of Redmine task #2556: Allow blank data values for optional covariates.
Bulk Upload: Added test for Redmine issue #2556: Check that no covariate row
gets inserted into the covariates table when an optional covariate value is left
blank.
Bulk Upload: Fix for Redmine issue #2556: Don't insert a covariate row if the data value given in the file is blank.
Bulk Upload: Changes to fixtures to get bulk upload controller tests to pass and
to facilitate data cleanup.  Removed 'pending' marker from bulk upload
controller tests.  Added missing data cleanup of entities and traits tables.