BUG: fix col iteration in DataFrame.round, #11611
Fixes #11611. The iterator now uses DataFrame.iteritems instead of direct indexing.

need some tests
 pls add the test example as well (you can just smoke tests it I think; e.g. assert it runs).

add a tests for passing a non-unique decimal.

put this in the bug-fix section for a whatsnew entry.
 Agreed.
 merged via https://github.com/pydata/pandas/commit/80a2d530bae32ef7ecf93aa275d17659ee8e8ac1

thanks!
