Enable case-insensitive string dissector tables

Permit passing TRUE as the parameter during table registration to achieve that
effect.

Use it in RTP media type table.

Bug: 10708
Change-Id: I892fb1a421d349f0c05197dec90f14fc34ad6b97
Reviewed-on: https://code.wireshark.org/review/5695
Reviewed-by: Evan Huus <eapache@gmail.com>
Petri-Dish: Evan Huus <eapache@gmail.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>