net/http: improve header parsing

[sago35]

This PR improves header parsing.

# before : Can't find "\r\n\r\n"
buf[ofs:ofs+n] = []byte{..., '\r', '\a', '\r'}
buf[ofs:ofs+n] = []byte{'\a', ...}

# after : Can find "\r\n\r\n"
buf[ofs:ofs+n] = []byte{..., '\r', '\a', '\r'}
buf[ofs-3:ofs+n] = []byte{'\r', '\a', '\r', '\a', ...} // 

I was aware of this problem when I tried to access the following

• https://httpbin.org/get
  • https://httpbin.org/ is very useful!

[deadprogram]

Thanks @sago35 now merging.