This allows httptest.ResponseRecorder to be wrapped and still maintain
its Flusher compatibility

"util" is a really bad name for a package since it isn't very
descriptive and so often collides with other names.

Unfortunately, this is a breaking change, but it's both very easy to fix
and perhaps more importantly also better to do now than later.

This makes it possible to show content length when writing the access log.

It turns out WriterProxy is pretty generally useful, especially when
defining custom http loggers. Expose it in a util package so that other
packages can use it.