Arithmetic operator on frames shows unexpected result

[zyzhu]

Example:

let df1 = frame [ "a" => Series.ofValues [ 1; 2]
                  "b" => Series.ofValues [ 3; 4] ]
let df2 = frame [ "b" => Series.ofValues [ 5; 6]
                  "c" => Series.ofValues [ 7; 8] ]
df1 + df2

I expect result shall be

     a         b  c         
0 -> <missing> 8  <missing> 
1 -> <missing> 10 <missing> 

But the current result is

     a b  c 
0 -> 1 8  7 
1 -> 2 10 8 

It's because of the implementation of Frame.zip left uncommon columns unmodified. https://github.com/fslaborg/Deedle/blob/master/src/Deedle/Frame.fs#L237

It is not intuitive as the operation shall happen only if both frame has value on that column. If only one of them has value, the output shall be left as missing showing the operator cannot be completed. That's the expected result from pandas.

df1 = pd.DataFrame([[1, 3], [2, 4]],
                    columns=['a', 'b'])
df2 = pd.DataFrame([[5, 7], [6, 8]],
                    columns=['b', 'c'])
df1 + df2

    a   b   c
0 NaN   8 NaN
1 NaN  10 NaN

[zyzhu]

Fixed in #434