This change set ensures that the protobuf representation for headers be in
bytes. This makes sure that if ever protobuf marshalling is non-deterministic,
we do not have problems because whenever we hash a header, we do that over
the serialized version we receive.

Change-Id: I838e0d5dec2f79f88fab63d92bdfb51d92c2f069
Signed-off-by: Alessandro Sorniotti <ale.linux@sopit.net>