try to optimize proxy function from `net.http` about guard

[jiajunhuang]

maybe I should read source code from here: https://github.com/valyala/fasthttp . here is the best practice from fasthttp:

Fasthttp best practices

• Do not allocate objects and []byte buffers - just reuse them as much
  as possible. Fasthttp API design encourages this.
• sync.Pool is your best friend.
• Profile your program
  in production.
  go tool pprof --alloc_objects your-program mem.pprof usually gives better
  insights for optimization opportunities than go tool pprof your-program cpu.pprof.
• Write tests and benchmarks for hot paths.
• Avoid conversion between []byte and string, since this may result in memory
  allocation+copy. Fasthttp API provides functions for both []byte and string -
  use these functions instead of converting manually between []byte and string.
  There are some exceptions - see this wiki page
  for more details.
• Verify your tests and production code under
  race detector on a regular basis.
• Prefer quicktemplate instead of
  html/template in your webserver.

[jiajunhuang]

I'm gonna handle this. but February is Chinese spring festival, I'll do it in March.

[jiajunhuang]

before March, I'm gonna doing these things for prepare:

• read HTTP RFC
• read implementation of fasthttp

[jiajunhuang]

done by #16