ocpayload: in DMV remove undesired trailing separator

ocpayload: in DMV remove undesired trailing separator

Also add more tests cases too.

Suggestion for improvements:

Note the test uses a different message container to prevent ambiguity:
 same one could be used but accessed at next indice
 because MessageContainer::setPayload
 actually adds given payload to internal list.

It would makes sense to rename method to MessageContainer::addPayload
 and eventually add a new different method like :
 MessageContainer::set(Payload) to reset list.

Bug: https://jira.iotivity.org/browse/IOT-1174
Change-Id: I9e7bf3c9eb7a311214e438ec2434d53b4ae0f313
Signed-off-by: Philippe Coval <philippe.coval@osg.samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/9069
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Ziran Sun <ziran.sun@samsung.com>