net/http: more tests, better comments, remove an allocation

Add more tests around the various orders handlers can access
and flush response headers.

Also clarify the documentation on fields of response and
chunkWriter.

While there, remove an allocation (a header clone) for simple
handlers.

benchmark                                   old ns/op    new ns/op    delta
BenchmarkServerFakeConnWithKeepAliveLite        15245        14966   -1.83%

benchmark                                  old allocs   new allocs    delta
BenchmarkServerFakeConnWithKeepAliveLite           24           23   -4.17%

benchmark                                   old bytes    new bytes    delta
BenchmarkServerFakeConnWithKeepAliveLite         1717         1668   -2.85%

R=golang-dev, gri
CC=golang-dev
https://codereview.appspot.com/8101043