HTTP headers are case insensitive handle them appropriately (#6390)

An issue was reproduced when minio-js client functional
tests are setting lower case http headers, in our current
master branch we specifically look for canonical host header
which may be not necessarily true for all http clients.
This leads to a perpetual hang on the *net.Conn*.

This PR fixes regression caused by #6206 by handling the
case insensitivity.

cmd/http/listener.go

@@ -94,8 +94,11 @@ func getResourceHost(bufConn *BufConn, maxHeaderBytes, methodLen int) (resource
 				continue
 			}
 
-			if strings.HasPrefix(token, "Host: ") {
-				host = strings.TrimPrefix(strings.TrimSuffix(token, "\r"), "Host: ")
+			// HTTP headers are case insensitive, so we should simply convert
+			// each tokens to their lower case form to match 'host' header.
+			token = strings.ToLower(token)
+			if strings.HasPrefix(token, "host: ") {
+				host = strings.TrimPrefix(strings.TrimSuffix(token, "\r"), "host: ")
 				return resource, host, nil
 			}
 		}

cmd/http/listener_test.go

@@ -425,9 +425,11 @@ func TestHTTPListenerAccept(t *testing.T) {
 	}{
 		{[]string{"localhost:0"}, nil, "GET / HTTP/1.0\r\nHost: example.org\r\n\r\n", "200 OK\r\n", "GET / HTTP/1.0\r\n"},
 		{[]string{nonLoopBackIP + ":0"}, nil, "POST / HTTP/1.0\r\nHost: example.org\r\n\r\n", "200 OK\r\n", "POST / HTTP/1.0\r\n"},
+		{[]string{nonLoopBackIP + ":0"}, nil, "HEAD / HTTP/1.0\r\nhost: example.org\r\n\r\n", "200 OK\r\n", "HEAD / HTTP/1.0\r\n"},
 		{[]string{"127.0.0.1:0", nonLoopBackIP + ":0"}, nil, "CONNECT \r\nHost: www.example.org\r\n\r\n", "200 OK\r\n", "CONNECT \r\n"},
 		{[]string{"localhost:0"}, tlsConfig, "GET / HTTP/1.0\r\nHost: example.org\r\n\r\n", "200 OK\r\n", "GET / HTTP/1.0\r\n"},
 		{[]string{nonLoopBackIP + ":0"}, tlsConfig, "POST / HTTP/1.0\r\nHost: example.org\r\n\r\n", "200 OK\r\n", "POST / HTTP/1.0\r\n"},
+		{[]string{nonLoopBackIP + ":0"}, tlsConfig, "HEAD / HTTP/1.0\r\nhost: example.org\r\n\r\n", "200 OK\r\n", "HEAD / HTTP/1.0\r\n"},
 		{[]string{"127.0.0.1:0", nonLoopBackIP + ":0"}, tlsConfig, "CONNECT \r\nHost: www.example.org\r\n\r\n", "200 OK\r\n", "CONNECT \r\n"},
 	}