inner_join on grouped_df loses factor level order

[wch]

Example:

input <- data.frame(g1 = factor(c('A','B','C'), levels = c('B','A','C')))
output <- data.frame(
  g1 = factor(c('A','B','C'), levels = c('B','A','C')),
  g2 = factor(c('A','B','C'), levels = c('B','A','C'))
)

res <- inner_join(group_by(input, g1), group_by(output, g1))
levels(res$g1)
# [1] "A" "B" "C"
levels(res$g2)
# [1] "B" "A" "C"

They both should be "B" "A" "C".