Copter: Use AHRS heading in GLOBAL_POSITION_INT message

The definition of the heading field for GLOBAL_POSITION_INT is the
compass heading, and so it is used for Plane and Rover.  Copter
however uses the GPS course over ground in this field.

My personal beef with this is simply that the mapping display in
MAVProxy is wrong for Copter.  The question is: Do any GCS currently
rely on this field for GPS course rather than getting it from
GPS_RAW_INT as they should?