Mode() not compatible with fillna()
I made an toy dataframe:
df = pandas.DataFrame([[1, 1, 1],[2, 1, 1],[2, 1, 1],[numpy.nan, numpy.nan, numpy.nan]], columns=["a","b","c"])

I try different methods to fill missing values. These work as expected:
df.fillna(df.mean())
df.fillna(df.median())

But this doesnt work:
df.fillna(df.mode())

Inspecting the output from df.mode() I see it has different format than df.mean() and df.median(). As I user I would expect the same behavior for these functions, and be able to fill missing values as described.
Using Pandas 0.15.2
