BUG: Fix problem with unique/single valued-columns

If a category in the mapping file had a unique/single valued column,
this would automatically be removed regardless of the fact that the user
could have asked to keep the column. Some tests were fixed to reflect
the case they were checking.

Fixes #271
TST: Add three more test cases related to #271
DOC: Add note about the fix of #271
Merge pull request #291 from ElDeveloper/issue-271

BUG: Fix issue #271