HTTP/2: fix $bytes_sent variable.
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.

Change-Id: Ib429799a7f2be46c3a45c4873c78fca668917fae
Signed-off-by: Piotr Sikora <ba6f3a1a6d7e1ecae91b31e93969cc392fbd0fdd@google.com>
Reviewed-on: https://nginx-review.googlesource.com/2820
Reviewed-by: Lizan Zhou <821ab109c028f980fd658b5ceedb38e095be5bc8@google.com>

firebase/nginx,firebase/nginx,firebase/nginx,firebase/nginx