Don't allocate for the Date or Content-Length headers.
A custom Date header formatter replaces use of time.Format.

benchmark                                   old ns/op    new ns/op    delta
BenchmarkClientServer                           67791        64424   -4.97%
BenchmarkClientServerParallel4                  62956        58533   -7.03%
BenchmarkClientServerParallel64                 62043        54789  -11.69%
BenchmarkServer                                254609       229060  -10.03%
BenchmarkServerFakeConnNoKeepAlive              17038        16316   -4.24%
BenchmarkServerFakeConnWithKeepAlive            14184        13226   -6.75%
BenchmarkServerFakeConnWithKeepAliveLite         8591         7532  -12.33%
BenchmarkServerHandlerTypeLen                   10750         9961   -7.34%
BenchmarkServerHandlerNoLen                      9535         8935   -6.29%
BenchmarkServerHandlerNoType                     9858         9362   -5.03%
BenchmarkServerHandlerNoHeader                   7754         6856  -11.58%

benchmark                                  old allocs   new allocs    delta
BenchmarkClientServer                              68           66   -2.94%
BenchmarkClientServerParallel4                     68           66   -2.94%
BenchmarkClientServerParallel64                    68           66   -2.94%
BenchmarkServer                                    21           19   -9.52%
BenchmarkServerFakeConnNoKeepAlive                 32           30   -6.25%
BenchmarkServerFakeConnWithKeepAlive               27           25   -7.41%
BenchmarkServerFakeConnWithKeepAliveLite           12           10  -16.67%
BenchmarkServerHandlerTypeLen                      19           18   -5.26%
BenchmarkServerHandlerNoLen                        17           15  -11.76%
BenchmarkServerHandlerNoType                       17           16   -5.88%
BenchmarkServerHandlerNoHeader                     12           10  -16.67%

Update #5195

R=nigeltao
CC=golang-dev
https://golang.org/cl/9432046