Deleting column used in grouping should remove grouped_df attribute

[puzzledvacuum]

If the user has a grouped_df and sets one of the coulumns used in the grouping to NULL this should remove the grouped_df attribute and make it a regular tbl.

Currently, if the user sets a column used as a grouping to NULL, and then tries to rename a different column, this causes the R session to abort.

Example:

df = data_frame(a = c(1,2,3), b = c(2,3,4), d = c(4,5,6))  
df = df %>% group_by(a,b)  
df$a = NULL  
class(df)
## [1] "grouped_df" "tbl_df"     "tbl"        "data.frame"
df = df %>% rename(a = d)

At this point the R session will crash.

[krlmlr]

Thanks, confirmed. Would you like to contribute a testthat test?

[hadley]

We don't need to override $ to remove the attribute, but we should throw an error rather than crashing.