filtering a grouped table removes indices attribute

For example:

``` R
# OK
iris %>% 
  group_by(Species) %>%
  attr("indices")

# not OK
iris %>% 
  group_by(Species) %>%
  filter(n() > 0) %>%
  attr("indices")

devtools::session_info()
# dplyr        0.4.0.9000 2015-01-11 Github (hadley/dplyr@cd91f33)    
```

If this is intended (I imagine its not), then _at least_ the `n_groups()` method for a `grouped_df` needs to be fixed.
