Error with `minor_breaks` and dates

[wch]

This example is taken from the visual tests.

set.seed(342)
df <- data.frame(
  date = seq(as.Date("2012-2-29"), len=100, by="1 day")[sample(100, 50)],
  price = runif(50)
)
df <- df[order(df$date), ]


# OK
qplot(date, price, data=df, geom="line") +
  scale_x_date(labels = date_format("%m/%d"), breaks = date_breaks("month"))

# Error
qplot(date, price, data=df, geom="line") +
  scale_x_date(labels = date_format("%m/%d"), breaks = date_breaks("month"),
    minor_breaks = date_breaks("week"))
# Error in Ops.Date((x - from[1]), diff(from)) : 
#  / not defined for Date objects

[wch]

The problem seems to be with scale_breaks_minor.date(). I think this function can be removed, and scale_breaks_minor.continuous() can be modified to handle things.