HTTP/3: support $server_protocol variable.

Now it holds "HTTP/3.0".  Previously it was empty.

src/http/ngx_http_parse.c

@@ -833,6 +833,10 @@ done:
         r->request_end = p;
     }
 
+    if (r->http_protocol.data) {
+        r->http_protocol.len = r->request_end - r->http_protocol.data;
+    }
+
     r->http_version = r->http_major * 1000 + r->http_minor;
     r->state = sw_start;
 

src/http/ngx_http_request.c

@@ -1139,10 +1139,6 @@ ngx_http_process_request_line(ngx_event_t *rev)
             r->method_name.len = r->method_end - r->method_start;
             r->method_name.data = r->method_start;
 
-            if (r->http_protocol.data) {
-                r->http_protocol.len = r->request_end - r->http_protocol.data;
-            }
-
             if (ngx_http_process_request_uri(r) != NGX_OK) {
                 break;
             }

src/http/v3/ngx_http_v3_request.c

@@ -110,6 +110,8 @@ ngx_http_v3_parse_request(ngx_http_request_t *r, ngx_buf_t *b)
             continue;
         }
 
+        ngx_str_set(&r->http_protocol, "HTTP/3.0");
+
         len = (r->method_end - r->method_start) + 1
             + (r->uri_end - r->uri_start) + 1
             + sizeof("HTTP/3") - 1;