Added content-encoding header

[douxxtech]

If no file is served, the Content-Encoding: identity header is not sent.

https://udn.realityripple.com/docs/Web/HTTP/Headers/Content-Encoding

[github-actions]

threads: 4 (auto), conns: 50, duration: 20s

[wrk]
Running 20s test @ http://127.0.0.1:8080/
  4 threads and 50 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     5.08ms   11.44ms 420.30ms   99.38%
    Req/Sec     1.71k   344.73     2.77k    73.25%
  136721 requests in 20.05s, 209.27MB read
  Socket errors: connect 0, read 1713, write 0, timeout 0
Requests/sec:   6820.17
Transfer/sec:     10.44MB