#1 dirty checking on byte array _contents_

[Willem van den Ende]

byte arrays are (unlike strings) mutable. dirty
checking in Mirage currently only regards the _identity
of the array attribute itself_ it does not check to see
if the _contents_ of the array have changed.

For the NCP problem this is not a problem, since byte
arrays are used only for images, which change in their
entirety.

It is not clear if this should be

- implemented
- implemented with a system property to switch it on or
off (performance will be _bad_ if we are going to check
large PDFs, images etc. for changes in their bytes).