The access log's response writer had no Unwrap method, so
http.ResponseController stopped at it and a handler's Flush or
SetWriteDeadline returned http.ErrNotSupported. It now has one.
With metrics on, the metrics library's std.Handler put a writer of its
own in the chain with the same gap. The Metrics middleware now calls
the library's Measure with a writer of ours that has Unwrap, and keeps
its place in the chain.
A new test sets a write deadline and flushes through the production
router over a real connection, with metrics off and on, on a global
route and in an admin page route group.
Model: opus-5-5
