rp.freq should also return NAs

[daroczig]

> rp.freq('game', rapport::ius2008)
         game   N          % Cumul. N  Cumul. %
1       never 352  51.994092      352  51.99409
2 very rarely 128  18.906942      480  70.90103
3      rarely  32   4.726736      512  75.62777
4   sometimes  60   8.862629      572  84.49040
5       often  37   5.465288      609  89.95569
6  very often  35   5.169867      644  95.12555
7      always  33   4.874446      677 100.00000
  **Missing**  ?
8       Total 677 100.000000      677 100.00000