Fix channel count when exporting RGB images in ImageJ

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/12091 and http://trac.openmicroscopy.org.uk/ome/ticket/11569.

To test, verify that both test cases outlined in the tickets are now working.  It would also be helpful to verify that exporting a non-RGB image with multiple channels still works as expected.