fix(esp_http_client): Fix invalid content length header

In case a request with no content is sent after one with the content
length header set the header of the previous request is sent with the
subsequent one.
For instance, an empty GET request after a PUT request will still
indicate the non-zero content length of the previous request.

This is fixed by clearing the content length header when it shouldn't be
set.

Author: chris-durand

components/esp_http_client/esp_http_client.c

@@ -1587,6 +1587,8 @@ static int http_client_prepare_first_line(esp_http_client_handle_t client, int w
                                       client->connection_info.method != HTTP_METHOD_DELETE);
         if (write_len != 0 || length_required) {
             http_header_set_format(client->request->headers, "Content-Length", "%d", write_len);
+        } else {
+            http_header_delete(client->request->headers, "Content-Length");
         }
     } else {
         esp_http_client_set_header(client, "Transfer-Encoding", "chunked");