It appears that http-serialiser is not working at all in 2.7.2. I
started with something relatively complicated and assumed it was my
mistake when it didn't work. But then I simplified until finally I tried
the example from the documentation related to that serialiser. Same
result - nothing is sent to the browser at all. The 'legacy'
xml-serialiser works fine.
I also notice that no examples shipped with 2.7.2 are using this
serialiser, so it seems possible that no one noticed. I tried debugging,
but got lost somewhere in the caching code. I'd appreciate it if someone
can have a look.