Some components in Spring assume (perhaps wrongly) that a channel
interceptor will not convert a mutable message into an immutable
one, and they continue to modify the headers dowstream. We can
dosge the issue of whether this is right or wrong by keeping the
message headers mutable, just in case.
It would probably be better to refactor the SpanInjector so that
it works with something other than the MessageBuilder, but we can
defer doing that in favour of this smaller change that works.
Adds a test for mutability. Also tested with the
gs-messaging-stomp-websocket guide from spring.io.
Fixes gh-276
1d1f28d971