Relates to #1925

**This is not ready to be merged. -- work in progress!**
Problem in #1925 was that we refactored and removed the outgoing message mutators from being invoked. See here:
https://github.com/Particular/NServiceBus/commit/33c8b81737cf7a768766bc76f8dc924c1aed8077#diff-1df1836ac4e373d2c1200c94c56ce852R48

Made changes after chatting with @SimonCropp.