hadley/dplyr — pull request #678 by matthieugomez (2014-10-09 22:39:23+00:00)
Title: Update order-by.R

I correct the following problem: attributes are lost when order_by is specified
````R
v<- c(1,1)
date <- c(1,2)
attr(v, "my_attribute") <- "This is a vector"
lag(v,1)
#> [1] NA 1
#> attr(,"my_attribute")
#> [1] "This is a vector"
lag(v,1, order_by = date)
#> [1] NA 1
````