content_encoding: ignore duplicate chunked encoding

- ignore duplicate "chunked" transfer-encodings from
  a server to accomodate for broken implementations
- add test1482 and test1483

Reported-by: Mel Zuser
Fixes #13451
Closes #13461

lib/content_encoding.c

@@ -1009,6 +1009,13 @@ CURLcode Curl_build_unencoding_stack(struct Curl_easy *data,
       }
 
       cwt = find_unencode_writer(name, namelen, phase);
+      if(is_transfer && cwt && strncasecompare(name, "chunked", 7) &&
+         Curl_cwriter_get_by_type(data, cwt)) {
+        /* A 'chunked' transfer encoding has already been added.
+         * Ignore duplicates. See #13451. */
+        return CURLE_OK;
+      }
+
       if(!cwt)
         cwt = &error_writer;  /* Defer error at use. */