TST: test for GH2623, object in frame upconverting a datetime64[ns] in another column
this is tests only at #2623 is covered by other PRS

You can feel free to push commits like this directly.
 yep, makes sense
 if you merge branches manually and push, please use `merge --no-ff`, to ensure a merge commit
that packages up related commits into a well-defined block. no need if it's something small.
 how is that different from pushing to my local branch then auto merging? which generates the merge commit
we do want the merge commit , right?
 no difference, it's just the command line equivalent of the greenbutton.
for a PR with serveral coupled commits, I prefer a merge commit yes.
But I often push singles straight to master.
 ok that's great thanks
