fix HTTP proxy OK response

[valture]

According to RFC 2616

HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
protocol elements except the entity-body (see appendix 19.3 for
tolerant applications).

[codecov]

Codecov Report

Merging #525 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #525   +/-   ##
=======================================
  Coverage   71.96%   71.96%           
=======================================
  Files         187      187           
  Lines        8528     8528           
=======================================
  Hits         6137     6137           
+ Misses       1847     1844    -3     
- Partials      544      547    +3

Impacted Files	Coverage Δ
proxy/http/server.go	56.47% <100%> (-1.18%)	⬇️
proxy/shadowsocks/server.go	60% <0%> (-4.35%)	⬇️
proxy/shadowsocks/client.go	74.46% <0%> (ø)	⬆️
app/proxyman/mux/mux.go	69.49% <0%> (+0.84%)	⬆️
proxy/socks/server.go	79.43% <0%> (+1.86%)	⬆️
common/buf/buffered_writer.go	88.57% <0%> (+2.85%)	⬆️
proxy/socks/client.go	82.6% <0%> (+2.89%)	⬆️

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