nghttpx: Don't emit :authority if request dones not contain authority info

RFC 7540 says that proxy should not emit :authority when translating
HTTP/1 request in origin or asterisk form to HTTP/2.  To keep this
semantics in tact, we should also refrain from emitting :authority if
it is missing (host header field is required in this case).

src/shrpx_downstream.h

@@ -127,7 +127,7 @@ struct Request {
       : fs(16), recv_body_length(0), unconsumed_body_length(0), method(-1),
         http_major(1), http_minor(1), upgrade_request(false),
         http2_upgrade_seen(false), connection_close(false),
-        http2_expect_body(false) {}
+        http2_expect_body(false), no_authority(false) {}
 
   void consume(size_t len) {
     assert(unconsumed_body_length >= len);
@@ -165,6 +165,10 @@ struct Request {
   // true if this is HTTP/2, and request body is expected.  Note that
   // we don't take into account HTTP method here.
   bool http2_expect_body;
+  // true if request does not have any information about authority.
+  // This happens when: For HTTP/2 request, :authority is missing.
+  // For HTTP/1 request, origin or asterisk form is used.
+  bool no_authority;
 };
 
 struct Response {

src/shrpx_http2_downstream_connection.cc

@@ -287,7 +287,7 @@ int Http2DownstreamConnection::push_request_headers() {
   // 1. :method
   // 2. :scheme
   // 3. :path
-  // 4. :authority
+  // 4. :authority (or host)
   // 5. via (optional)
   // 6. x-forwarded-for (optional)
   // 7. x-forwarded-proto (optional)
@@ -300,8 +300,6 @@ int Http2DownstreamConnection::push_request_headers() {
   nva.push_back(
       http2::make_nv_lc_nocopy(":method", http2::to_method_string(req.method)));
 
-  nva.push_back(http2::make_nv_lc_nocopy(":authority", authority));
-
   if (req.method != HTTP_CONNECT) {
     assert(!req.scheme.empty());
 
@@ -312,6 +310,14 @@ int Http2DownstreamConnection::push_request_headers() {
     } else {
       nva.push_back(http2::make_nv_ls_nocopy(":path", req.path));
     }
+
+    if (!req.no_authority) {
+      nva.push_back(http2::make_nv_lc_nocopy(":authority", authority));
+    } else {
+      nva.push_back(http2::make_nv_lc_nocopy("host", authority));
+    }
+  } else {
+    nva.push_back(http2::make_nv_lc_nocopy(":authority", authority));
   }
 
   http2::copy_headers_to_nva_nocopy(nva, req.fs.headers());

src/shrpx_http2_upstream.cc

@@ -298,6 +298,7 @@ int Http2Upstream::on_request_headers(Downstream *downstream,
 
   // nghttp2 library guarantees either :authority or host exist
   if (!authority) {
+    req.no_authority = true;
     authority = req.fs.header(http2::HD_HOST);
   }
 

src/shrpx_https_upstream.cc

@@ -308,6 +308,8 @@ int htp_hdrs_completecb(http_parser *htp) {
         return -1;
       }
 
+      req.no_authority = true;
+
       if (method == HTTP_OPTIONS && path == "*") {
         req.path = "";
       } else {