BUG/MINOR: cache/htx: Be sure to count partial trailers

When a chunked object is served from the cache, If the trailers are not pushed
in the channel's buffer in one time, we still have to count them in the total
written bytes in the buffer.

This patch must be backported to 1.9.
diff --git a/src/cache.c b/src/cache.c
index d80ee90..77e411f 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -1027,12 +1027,12 @@
 		unsigned int len = first->len - sizeof(*cache_ptr) - appctx->ctx.cache.sent;
 
 		ret = htx_cache_dump_data(appctx, res_htx, HTX_BLK_TLR, len);
+		total += ret;
 		if (ret < len) {
 			si_rx_room_blk(si);
 			goto out;
 		}
 
-		total += ret;
 		if (first->len == sizeof(*cache_ptr) + appctx->ctx.cache.sent)
 			appctx->st0 = HTX_CACHE_EOM;
 	}