BUG: Common NumericIndex.__new__, with fixed name handling
closes #12309 

pls add tests for the name propogation
 Ping?
(in general, are you postponing non-0.18.0-tagged PRs to after the 0.18.0 release?)
 @toobaz yes
 though non-api changes or straightforward stuff I will merge (so pinging is good)
 @toobaz if you'd like to rebase / update, pls reopen.
 Rebased, updated, addressed your comments, except a couple of them which were unclear to me. I don't think I am allowed to reopen.

While extending `est_ensure_copied_data` as you suggested, a bug came out in `DateTimeIndex`, now fixed.
 (not sure why the code isn't being updated... shall I open a new PR?)
 @toobaz yes, you will have to open a new PR. That's something with github that you cannot reopen a PR once the branch has been updated while the PR is closed (so we should have reopened it before you pushed updated to the branch ..). Anyway, no problem to open a new one! :-)
