BUG: Series.fillna() raises if given a numerically convertible string
First reported here but it's been dormant for a long time: https://github.com/pydata/pandas/pull/9043 

I think it'd be good to patch this bug. @jreback please let me know which release this should go to and I can add a release note. Thanks. 

this looks ok for 0.16.1.
 sounds good, I added a release note 
 ok, ping on green.
 @jreback green now 
 thanks!
