frame in `ggplotly` not working when taking a `Date` object

Hi Carson,

The new animation feature is awesome. But I run into an issue when the frame arg takes a Date object, and I need to convert to numerics. Please see the little reproducible example below:

library(plotly)
df <- data.frame(
  Date = seq(as.Date("2017-01-01"), as.Date("2017-01-05"), by = 1),
  Hour = rep(1:24, 5),
  Value = rnorm(24 * 5)
)
p1 <- ggplot(aes(Hour, Value, frame = Date), data = df) +
  geom_line() # not working
ggplotly(p1)
p2 <- ggplot(aes(Hour, Value, frame = as.numeric(Date)), data = df) +
  geom_line() # working
ggplotly(p2)

Thanks a lot,
Earo

Run the reproducible R example from the issue and compare ggplotly(p1), where frame uses Date, with ggplotly(p2), where it uses as.numeric(Date). Start at ggplotly's handling of the frame aesthetic and trace how Date values are converted. Done means a Date-valued frame works without manual numeric conversion and the existing numeric case remains working.