net/http: Release reference to chunkWriter's bufio.Writer on hijack
When a connection is hijacked, release the reference to the bufio.Writer
that is used with the chunkWriter. The chunkWriter is not used after
the connection is hijacked.
Also add a test to check that double Hijack calls do something sensible.
benchmark old ns/op new ns/op delta
BenchmarkServerHijack 24137 20629 -14.53%
benchmark old allocs new allocs delta
BenchmarkServerHijack 21 19 -9.52%
benchmark old bytes new bytes delta
BenchmarkServerHijack 11774 9667 -17.90%
R=bradfitz, dave, chris.cahoon
CC=golang-codereviews
https://golang.org/cl/
39440044