BUG: replace with a scalar works like a list of to_replace for compat with 0.12 (GH5319)
Merge pull request #5600 from jreback/replace_api

BUG: replace with a scalar works like a list of to_replace for compat with 0.12 (GH5319)
ENH: Make set_option into a contextmanager that undos itself on __exit__