Add proper decoding for type field in OBEX headers

Change type field type from byte sequence to string.

Change-Id: I3e2ac6fbb5ce7187ba1183d63236e9a2bcd824ee
Reviewed-on: https://code.wireshark.org/review/4417
Petri-Dish: Michal Labedzki <michal.labedzki@tieto.com>
Tested-by: Michal Labedzki <michal.labedzki@tieto.com>
Reviewed-by: Michal Labedzki <michal.labedzki@tieto.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>