Series get function returns None instead of NaN on missing key
According to the documentation (Intro to Data Structures), the correct behavior is:

s.get('f')
nan

however, when 'f' is not a key of a Series, the get function returns None, as in:

s.get('f') is None
True

A workaround is to use get(..., NaN), as in:

s.get('f', NaN)
nan
