BUG: fix for interp with axis=1 and inplace
Closes https://github.com/pydata/pandas/issues/9687

There's still some weird issues with dtypes. In my test case there's a DataFrame with dtypes [int, float, int]. After the interpolation they should be downcasts to [int, int, int] when the missing value is filled. But along the way a transpose changes things to `[float, float, float]` before filling. And then (I think) `self._update_inplace` doesn't change dtypes, so the actual result is `[float, float, float]`.
