New implementation of TimeoutHandler: buffer everything to memory.
All or nothing: either the handler finishes completely within the
timeout (in which case the wrapper writes it all), or it misses the
timeout and none of it gets written, in which case handler wrapper can
reliably print the error response without fear that some of the
wrapped Handler's code already wrote to the output.
Now the goroutine running the wrapped Handler has its own write buffer
and Header copy.