http: use strict mode for setHeader

When I profile http server module, I find `setHeader()` use 3% self time in benchmark test. Although V8 will optimize it, but it take too many time. This patch speed up the function ~10%.

This change effect http benchmark very little. So I use this benchmark to present result: <https://gist.github.com/JacksonTian/4849b7c3845eec56bc9e> .

Anyway, the strict mode is amazing.