do() with grouped data crashes R session when returned data frame contains a list

[wch]

Example:

# Returns a data frame where one column is a list containing a numeric vector
foo <- function(x) {
  res <- data.frame(a=1, b=2)
  res$c <- list(c(5,6,7))
  res
}
foo()

# Not grouped: OK
data.frame(g=1) %>% do(foo(.))

# Grouped: crashes R
data.frame(g=1) %>% group_by(g) %>% do(foo(.))

This is needed for compute_boxplot in ggvis.

[hadley]

Duplicate of #463