summarise and group_by doesn't work with only one grouping variable with a tbl_cube

`summarise` and `group_by` works on two grouping variables, but not one, when using a tbl_cube:

``` r
summarise(group_by(nasa,month,year),temp=mean(temperature))
Source: local array [72 x 2]
D: month [int, 12]
D: year [int, 6]
M: temp [dbl[12,6]]

summarise(group_by(nasa,month),temp=mean(temperature))
Error: Invalid input
```
