Using dplyr functions on grouped data frame with variable of class yearmon generates error

[millerjef]

I believe this issue is similar to resolved issue #390.

When using dplyr::group_by on a data frame containing a variable of class yearmon (zoo package), then I get:
Error: column 'yearMon' has unsupported type : yearmon

This occurs irrespective of whether the variable with class yearmon is one of the variables on which the data is to be grouped. I would also like to be able to group_by other variables with classes as defined by the zoo package (e.g., yearmon, yearqtr, etc.).

Please see minimum reproducible example:

library(zoo)
library(lubridate)
library(dplyr)

dataByDateAndLetter <- data.frame(
  time = sample(as.Date('2009-01-01') + 0:11, size = 20, replace = TRUE),
  letter = sample(c("a","b","c"), size = 20, replace = TRUE),
  x = rnorm(10,0,1)
)

dataByDateAndLetter <- mutate(dataByDateAndLetter, 
                                        time=ymd(time),
                                        yearMon=as.yearmon(time),
                                        yearQtr=as.yearqtr(time))

summaryDataByLetter <- dataByDateAndLetter %>%
  group_by(letter) %>% summarise(minX = min(x))

My environment (yes, I'm using a fairly old unstable build).

sessionInfo()
R Under development (unstable) (2014-09-17 r66626)
Platform: x86_64-apple-darwin13.1.0 (64-bit)

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] tidyr_0.2.0     lubridate_1.3.3 zoo_1.7-11      dplyr_0.4.1    

loaded via a namespace (and not attached):
 [1] assertthat_0.1   DBI_0.3.1        digest_0.6.8     evaluate_0.5.5   formatR_1.0      grid_3.2.0       knitr_1.6       
 [8] lattice_0.20-30  lazyeval_0.1.10  magrittr_1.5     memoise_0.2.1    nycflights13_0.1 parallel_3.2.0   plyr_1.8.1      
[15] R6_2.0           Rcpp_0.11.3      stringr_0.6.2    tools_3.2.0     

[hadley]

Now part of #1105