Fix aggregateRowsBy in Issue 375

[zyzhu]

Attempt to fix #375

1. Get all values of groupBy including missing values
2. Added test

sample:
     A         B C 
1 -> 10        1 1 
2 -> <missing> 2 2 
3 -> 10        1 3 
4 -> <missing> 2 4 
5 -> 10        3 5

operation:
sample |> Frame.aggregateRowsBy ["A";"B"] ["C"] Stats.mean

expected:
     A         B C 
0 -> 10        1 2 
1 -> <missing> 2 3 
2 -> 10        3 5