BUG: GH12071 .reset_index() should create a RangeIndex
closes #12071 

Fixes #12071 
 add this issue number to right after #939 in `whatsnew/v0.18.0`
 Thanks. Updated the pull request. Not sure if I put the reference to the issue in the right place in "whatsnew" (this is my first PR to pandas).
 add a test for Series as well
 merged via https://github.com/pydata/pandas/commit/16a7799702f43d58ee2c2b3582c5a2c92b34fc0c

thanks!
 Thanks for the help @jreback 
