TST: Add test case for 0.12 regression with loc from GH4825
Closes #4825.

thanks
let me see where this is coming from in any event
 @jreback btw, should this raise?

``` python
ser = Series([0.2, 1, 2])
ser.loc[[3, 3]]
```
 @jreback so I think this is unnecessary now, right?
 yep I incorporated them (slightly modified)
