fix(proxy): keep request header host if user set it while using proxy

[scplay]

If user set a customized header host field, the host should not be changed while sending request.

However, when using proxy config and default adapter http in nodejs, the host header will be reset to request url no matter config headers.host field is set or not.

example:

axios.get("http://127.0.0.1:4000", {
  headers: {
    host: "example.com",
  },
  proxy: {
    protocol: "http",
    host: "127.0.0.1",
    port: 8888,
  },
});

proxy request should keep header host field as example.com instead of 127.0.0.1:4000

before:

GET / HTTP/1.1
host 127.0.0.1:4000

fix:

GET / HTTP/1.1
host example.com