gRPC: special handling of the TE request header. According to the gRPC protocol specification, the "TE" header is used to detect incompatible proxies, and at least grpc-c server rejects requests without "TE: trailers". To preserve the logic, we have to pass "TE: trailers" to the backend if and only if the original request contains "trailers" in the "TE" header. Note that no other TE values are allowed in HTTP/2, so we have to remove anything else.

--- a/src/http/ngx_http_request.h
+++ b/src/http/ngx_http_request.h
@@ -197,6 +197,7 @@ typedef struct {
     ngx_table_elt_t                  *if_range;
 
     ngx_table_elt_t                  *transfer_encoding;
+    ngx_table_elt_t                  *te;
     ngx_table_elt_t                  *expect;
     ngx_table_elt_t                  *upgrade;