BUG-1065: translation of Ipv6ExtHeader flags to int

- moved translation to separate method + test
- fixed the false part of ternary operator

(cherry picked from commit 4b7d8d11de20dc6c0a22b7d0e305ae25ead39d1e)

Change-Id: I498dbf6c37f2d7348743d33bd49fe2ff46870621
Signed-off-by: Michal Rehak <mirehak@cisco.com>