please merge PROTON-560 (fix incorrect frame size calculation) to omdi-proton-0.5

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1587214 13f79535-47bb-0310-9956-ffa450edef68
(cherry picked from commit a9932a2bf2b58d89300d0a4ae63e02a7397c72c3)

Conflicting changes to tests/python/proton_tests/ssl.py discarded.