[C++] Put custom header prefix before header key
[C++] Put custom header prefix before header key

Change-Id: I74fb9c7261d8c1078d888a7d4284abc1b9d63c1f

bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr