selector test: add test for resetting columns
Selection: don't bother calling allowSelect for deselection

This avoids an issue with resetting columns while something is selected;
it also short-circuits unnecessary logic, since if it's not possible to
select a row, it should always be deselected anyway.
Note Selection deselect/allowSelect fix in CHANGES.md
Merge branch 'master' of https://github.com/gratex/dgrid