HTTP/2: fix $bytes_sent variable.

Previously, its value accounted for payloads of HEADERS, CONTINUATION
and DATA frames, as well as frame headers of HEADERS and DATA frames,
but it didn't account for frame headers of CONTINUATION frames.

Signed-off-by: Piotr Sikora <piotrsikora@google.com>
diff --git a/src/http/v2/ngx_http_v2_filter_module.c b/src/http/v2/ngx_http_v2_filter_module.c
index 42fa0c5..8080e83 100644
--- a/src/http/v2/ngx_http_v2_filter_module.c
+++ b/src/http/v2/ngx_http_v2_filter_module.c
@@ -769,6 +769,8 @@
         rest -= frame_size;
 
         if (rest) {
+            frame->length += NGX_HTTP_V2_FRAME_HEADER_SIZE;
+
             type = NGX_HTTP_V2_CONTINUATION_FRAME;
             flags = NGX_HTTP_V2_NO_FLAG;
             continue;