DICOM: also use the pixel sign tag to detect the pixel type

Closes http://trac.openmicroscopy.org.uk/ome/ticket/12568.

To test, verify that the files from QA 9532 and QA 9552 have pixel type int16, and have a black background/white artifact when imported.  Without this change, the pixel type should have been uint16, and the images had a white background/black artifact.