Fix DataFrame.size to consider its number of columns.

[ueshin]

DataFrame.size should consider its number of columns:

>>> ks.Series({'a': 1, 'b': 2, 'c': None}).size
3
>>> ks.DataFrame({'col1': [1, 2, None], 'col2': [3, 4, None]}).size
3
>>> ks.DataFrame(index=[1, 2, None]).size
3

This should be:

>>> pd.Series({'a': 1, 'b': 2, 'c': None}).size
3
>>> pd.DataFrame({'col1': [1, 2, None], 'col2': [3, 4, None]}).size
6
>>> pd.DataFrame(index=[1, 2, None]).size
0