Merged zIndex fix from GitHub issue

https://github.com/eternicode/bootstrap-datepicker/issues/1129

Once it is officially merged, we can revert back to the original.
single date mode wasn't handled correctly

(cherry picked from commit 6e3ba6682d2e5b31c20ef37d087ffce06940ec07)
never deselect the date on Enter even in multidate mode

(cherry picked from commit 4ebbf2d41fe7f8aa4bfe116a81717f160e348d97)