Provide correct contenttype after a redirect. Needed by abaco.

Reference: /n/sources/patch/applied/webfs-redir-contenttype
Date: Sat Apr  8 11:48:04 CES 2006
Signed-off-by: uriel@cat-v.org

--- /sys/src/cmd/webfs/http.c	Sat Apr  8 11:42:44 2006
+++ /sys/src/cmd/webfs/http.c	Sat Apr  8 11:42:36 2006
@@ -37,8 +37,9 @@
 static void
 contenttype(HttpState *hs, char *value)
 {
-	if(hs->c->contenttype == nil)
-		hs->c->contenttype = estrdup(value);
+	if(hs->c->contenttype != nil)
+		free(hs->c->contenttype);
+	hs->c->contenttype = estrdup(value);
 }
 
 static void