complete() turns ordered factors into unordered factors

[phirsch]

If complete() is applied to a column that is an ordered factor, the result is no longer an ordered factor, but a plain factor instead (in tidyr_0.4.1):

> df = data_frame(a = as.ordered(1:2), b = as.ordered(1:2))
> str(df)
Classes ‘tbl_df’, ‘tbl’ and 'data.frame':   2 obs. of  2 variables:
 $ a: Ord.factor w/ 2 levels "1"<"2": 1 2
 $ b: Ord.factor w/ 2 levels "1"<"2": 1 2
> df %>% complete(a, b) %>% str()
Classes ‘tbl_df’, ‘tbl’ and 'data.frame':   4 obs. of  2 variables:
 $ a: Factor w/ 2 levels "1","2": 1 1 2 2
 $ b: Factor w/ 2 levels "1","2": 1 2 1 2
>