net/http: reduce per-request allocations with responseW...

net/http: reduce per-request allocations with responseWriter wrapper

Only generate 8 bytes of garbage ResponseWriter garbage per request,
reusing the large state between requests.

benchmark                         old ns/op    new ns/op    delta
BenchmarkServerHandlerTypeLen          8548         8768   +2.57%
BenchmarkServerHandlerNoLen            7592         7738   +1.92%
BenchmarkServerHandlerNoType           8110         8435   +4.01%
BenchmarkServerHandlerNoHeader         5834         5931   +1.66%

benchmark                        old allocs   new allocs    delta
BenchmarkServerHandlerTypeLen            17           17    0.00%
BenchmarkServerHandlerNoLen              14           14    0.00%
BenchmarkServerHandlerNoType             15           15    0.00%
BenchmarkServerHandlerNoHeader            9            8  -11.11%

benchmark                         old bytes    new bytes    delta
BenchmarkServerHandlerTypeLen          1881         1726   -8.24%
BenchmarkServerHandlerNoLen            1838         1683   -8.43%
BenchmarkServerHandlerNoType           1845         1690   -8.40%
BenchmarkServerHandlerNoHeader         1029          807  -21.57%

Update Issue 5195