net/textproto: adjust initial MimeHeader map size

net/textproto: adjust initial MimeHeader map size

** Not for submission ***

lucky(~/go/src/pkg/net/http) % ~/go/misc/benchcmp {old,new}.txt
benchmark                          old ns/op    new ns/op    delta
BenchmarkReadRequestChrome              9952         9101   -8.55%
BenchmarkReadRequestCurl                4316         4607   +6.74%
BenchmarkReadRequestApachebench         4399         4667   +6.09%
BenchmarkReadRequestSiege               6381         6216   -2.59%
BenchmarkReadRequestWrk                 2803         3337  +19.05%

benchmark                           old MB/s     new MB/s  speedup
BenchmarkReadRequestChrome             61.39        67.13    1.09x
BenchmarkReadRequestCurl               18.07        16.93    0.94x
BenchmarkReadRequestApachebench        18.64        17.57    0.94x
BenchmarkReadRequestSiege              24.29        24.93    1.03x
BenchmarkReadRequestWrk                14.27        11.99    0.84x

benchmark                         old allocs   new allocs    delta
BenchmarkReadRequestChrome                32           30   -6.25%
BenchmarkReadRequestCurl                  15           15    0.00%
BenchmarkReadRequestApachebench           16           16    0.00%
BenchmarkReadRequestSiege                 22           21   -4.55%
BenchmarkReadRequestWrk                   11           11    0.00%

benchmark                          old bytes    new bytes    delta
BenchmarkReadRequestChrome              3148         3411    8.35%
BenchmarkReadRequestCurl                 906         2203  143.16%
BenchmarkReadRequestApachebench          956         2252  135.56%
BenchmarkReadRequestSiege               1398         2425   73.46%
BenchmarkReadRequestWrk                  757         2143  183.09%

[bradfitz, 2013-02-09 01:12:10]

I have a patch lying around somewhere that counted the number of '\n' bytes and
used that size.  I forget the results.  Maybe they weren't good if I didn't send
it.