Replaced all xs_dict_iter() with xs_dict_next().

http.c

@@ -77,7 +77,8 @@ xs_dict *http_signed_request_raw(const char *keyid, const char *seckey,
 
     /* transfer the original headers */
     hdrs = xs_dict_new();
-    while (xs_dict_iter(&headers, &k, &v))
+    int c = 0;
+    while (xs_dict_next(headers, &k, &v, &c))
         hdrs = xs_dict_append(hdrs, k, v);
 
     /* add the new headers */

xs.h

@@ -1028,6 +1028,9 @@ int xs_dict_iter(xs_dict **dict, xs_str **key, xs_val **value)
 int xs_dict_next(const xs_dict *dict, xs_str **key, xs_val **value, int *ctxt)
 /* iterates a dict, with context */
 {
+    if (xs_type(dict) != XSTYPE_DICT)
+        return 0;
+
     int goon = 1;
 
     char *p = (char *)dict;