support for POSIXct classes in image.default axes

Hello, I would like to suggest the following change to image.default in
src\library\graphics\R\image.R:

98c98
<       plot(NA, NA, xlim = xlim, ylim = ylim, type = "n", xaxs = xaxs,
---

       plot(x[1], y[1], xlim = xlim, ylim = ylim, type = "n", xaxs = xaxs,

This provides all the support of axis.POSIXt that plot.default gives,
currently the default new plot in image.default uses NAs and the axis class
is ignored. This is nice for plotting time series data stored in a matrix.

Here's a dummy example that shows usage with my change:

data(volcano)
x <- list(x = Sys.time() + seq(1, 1e6, length = nrow(volcano)), y =
1:ncol(volcano), z = volcano)

## date-time formatting on the x-axis
image(x)

## date-time formatting on the y-axis
image(x$y, x$x, t(x$z))

Without the change we get the very large underlying numeric values for the
times on the axis.

We can still override the default axis to do a workaround: :
image(x, axes = FALSE)
axis.POSIXct(x$x, side = 1)

Cheers, Mike.