Fixes #6822/bz1117554 - Tasks search breaks for defualt searches

Prior to this commit
Task.search_for("foo") would search with an invalid whereclause
Something like
"""....
OR foreman_tasks_locks."" ILIKE 'Red' OR users."" ILIKE 'Red' OR
users."" ILIKE 'Red')
"""

This commit fixes that by hard coding the column names to search on if
none are provided.
Merge pull request #82 from parthaa/filter-error

Fixes #6822/bz1117554 - Tasks search breaks for defualt searches