Missing `Host` header causes signature mismatch

The 7th draft of the "Signing HTTP Messages" spec [recommends](https://tools.ietf.org/html/draft-cavage-http-signatures-07#appendix-C.2) the use of the `Host` header when calculating a signature.
However, it looks like `Request` from `net/http` removes `Host` from the headers and instead makes it available on the `Request` structure itself.

Example:
```
&http.Request{
Method:"GET",
URL:(*url.URL)(0xc42010d100),
Proto:"HTTP/1.1",
ProtoMajor:1,
ProtoMinor:1,
Header:http.Header{
"Date":[]string{"Tue, 03 Oct 2017 23:18:06 GMT"},
"Accept-Encoding":[]string{"gzip;q=1.0,deflate;q=0.6,identity;q=0.3"},
"Signature":[]string{"keyId=\"key1\",algorithm=\"hmac-sha256\",headers=\"(request-target) host date\",signature=\"T09YOpz+vN+VltcLRok8NgZSVSgm4W6EhjRUNwX7JXQ=\""},
"Connection":[]string{"keep-alive"},
"Keep-Alive":[]string{"30"},
"Accept":[]string{"application/x-msgpack"},
"User-Agent":[]string{"The Client"}
},
Body:http.noBody{},
GetBody:(func() (io.ReadCloser, error))(nil),
ContentLength:0,
TransferEncoding:[]string(nil),
Close:false,
Host:"localhost:3500",
Form:url.Values(nil),
PostForm:url.Values(nil),
MultipartForm:(*multipart.Form)(nil),
Trailer:http.Header(nil),
RemoteAddr:"[::1]:53522",
RequestURI:"/v1/files?ids%5B%5D=1",
TLS:(*tls.ConnectionState)(nil),
Cancel:(<-chan struct {})(nil),
Response:(*http.Response)(nil),
ctx:(*context.cancelCtx)(0xc4201ff8c0)
}
```
Should the `Host` header be special-cased?

The issue is about the net/http Request struct in Go, which moves the Host header to a field. Look at the library's signature calculation code to see how headers are processed. The fix likely involves checking the Request.Host field and including it in the signature when the 'host' header is listed. Review the HTTP signatures spec draft and existing tests to understand the expected behavior.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.