HTTP Header are redundantly mapped to SY Context Properties with Scope MESSAGE and EXCHANGE

Issue

We use HTTP header to transport governance metadata.

When the headers are mapped into SwitchYard they are persisted into context properties with scope MESSAGE and EXCHANGE. This is inconsistent to the documentation on https://docs.jboss.org/author/display/SWITCHYARD/Message+Composition. We would expect the header only to be present in the MESSAGE scope.

Furthermore when these properties are then remapped into the HTTP message we get inconsistent behaviour. Sometimes the values are read from the MESSAGE scope and sometimes from the EXCHANGE scope.