Fillna df.mean() compatibility test

[xdssio]

Desired functionality for fillna in a standard use case.

# Current implementation
df['x'] = df.x.fillna(float(df.x.mean()))

vs

# Desired functionality
df['x'] = df.x.fillna(df.x.mean())

• implement new functionality
• add unit-test
• review

[maartenbreddels]

Yeah, that should work indeed, I think. Will look into it.

[maartenbreddels]

This was not a specific problem with fillna/mean, but with and function added to a dataframe. I've made the test and code more general.