relay/api: combine request headers with the same name

If a request repeats the same header name multiple times, merge the
header values into a comma separated string. Previously, only the last
header specified would be used.

For header fields that are defined as a comma-separated list, a client
may choose to send it as multiple headers instead of one header with
comma-separated values. The specification says that these are
equivalent, so we can therefore join the headers into a comma-separated
string.

This is specified at https://datatracker.ietf.org/doc/html/rfc7230#section-3.2.2
which says:

    A sender MUST NOT generate multiple header fields with the same field
    name in a message unless either the entire field value for that
    header field is defined as a comma-separated list [i.e., #(values)]
    or the header field is a well-known exception (as noted below).

    A recipient MAY combine multiple header fields with the same field
    name into one "field-name: field-value" pair, without changing the
    semantics of the message, by appending each subsequent field value to
    the combined field value in order, separated by a comma.  The order
    in which header fields with the same field name are received is
    therefore significant to the interpretation of the combined field
    value; a proxy MUST NOT change the order of these field values when
    forwarding a message.

src/plugins/relay/relay-http.c

@@ -385,7 +385,7 @@ relay_http_parse_header (struct t_relay_http_request *request,
                          int ws_deflate_allowed)
 {
     char *pos, *name, *name_lower, *error, **items;
-    const char *ptr_value;
+    const char *existing_value, *ptr_value;
     int i, num_items;
     long number;
 
@@ -424,6 +424,10 @@ relay_http_parse_header (struct t_relay_http_request *request,
         ptr_value++;
     }
 
+    existing_value = weechat_hashtable_get (request->headers, name_lower);
+    if (existing_value)
+        ptr_value = WEECHAT_STR_CONCAT(", ", existing_value, ptr_value);
+
     /* add header in the hashtable */
     weechat_hashtable_set (request->headers, name_lower, ptr_value);