Hello, I'm webkit developer in South Korea.

In case, Content-Type has "" which value is empty string, not NULL.
libsoup set "Content-Length: 0" forcely.

I think that below change is proper to handle Content-Type.

soup-message.c (any version is same)
void
soup_message_set_request (SoupMessage    *msg,
			  const char     *content_type,
			  SoupMemoryUse   req_use,
			  const char     *req_body,
			  gsize           req_length)
{
	g_return_if_fail (SOUP_IS_MESSAGE (msg));
	g_return_if_fail (content_type != NULL || req_length == 0);

--	if (content_type) {
++	if (content_type && *content_type != 0) {
		soup_message_headers_replace (msg->request_headers,
					      "Content-Type", content_type);
		soup_message_body_append (msg->request_body, req_use,
					  req_body, req_length);
	} else {
		soup_message_headers_remove (msg->request_headers,
					     "Content-Type");
		soup_message_body_truncate (msg->request_body);
	}
}

On libsoup port of webkit, this problem is reproduced when <object> include a server flash resource with parameter.

GET /static_v8/flash/playerlist-nct_121010.swf HTTP/1.1
Host: stc.nct.nixcdn.com
User-Agent: Mozilla/5.0 (SMART-TV; X11; Linux i686) AppleWebKit/534.7 (KHTML, like Gecko) Version/5.0 Safari/534.7
Content-Type: <----- mime-type is not defined from HTML content
Accept: */*
Accept-Encoding: gzip
Content-Length: 0 <---- Content-Length is set forcely