Values of missingval still shown in the overview of the Raster

[felixcremer]

I am still working with the data from #605 and I am constructing a Raster object from the Netcdf subgroup. Setting the missingval keyword seems to have no effect and I can still see the missingval in the array when the Raster is shown.

This is a reproducer with small data:

arr = rand(10,10)
r = Raster(arr, (X(1:10,Y(1:10)), missingval=arr[1,1])

I would have expected that the first element of the Raster is shown as missing. skipmissing works as expected.