net/http: more request benchmarks

Add benchmarks for common http benchmarking tools. The intent is to catch optimisations which favor synthetic benchmarks that do not show improvements for real clients like Chrome.

BenchmarkReadRequestChrome        200000             10133 ns/op          60.29 MB/s        3148 B/op         32 allocs/op
BenchmarkReadRequestCurl          500000              4314 ns/op          18.08 MB/s         905 B/op         15 allocs/op
BenchmarkReadRequestApachebench   500000              4363 ns/op          18.79 MB/s         956 B/op         16 allocs/op
BenchmarkReadRequestSiege         500000              6408 ns/op          24.19 MB/s        1397 B/op         22 allocs/op
BenchmarkReadRequestWrk          1000000              2838 ns/op          14.09 MB/s         757 B/op         11 allocs/op

R=golang-dev, bradfitz
CC=golang-dev, haimuiba
https://codereview.appspot.com/7300075
spec: clarify that any unsafe.Pointer type is okay in conversion

The spec is not clear about whether this is allowed or not,
but both compilers allow it, because the reflect implementation
takes advantage of it. Document current behavior.

Fixes issue 4679.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7303064