MINOR: mux-h1: parse the content-length header on output and set H1_MF_CLEN

The H1_MF_CLEN flag is needed to figure whether a content-length header is
present or not when producing a request, so let's check it on output just
like we already check the transfer-encoding header.
diff --git a/src/mux_h1.c b/src/mux_h1.c
index 4df1289..5693739 100644
--- a/src/mux_h1.c
+++ b/src/mux_h1.c
@@ -1538,6 +1538,10 @@
 
 				if (isteqi(n, ist("transfer-encoding")))
 					h1_parse_xfer_enc_header(h1m, v);
+				else if (isteqi(n, ist("content-length"))) {
+					if (h1_parse_cont_len_header(h1m, &v) <= 0)
+						goto skip_hdr;
+				}
 				else if (isteqi(n, ist("connection"))) {
 					h1_parse_connection_header(h1m, v);
 					h1_process_conn_mode(h1s, h1m, NULL, &v);