Avoid multiple reallocations when receive a big buffer in post method call.

[Dwordcito]

Memory optimization in POST method handling

• Improved buffer management to avoid multiple reallocations when processing large POST requests
• This change reduces memory fragmentation and improves performance for applications that handle substantial POST data

[Dwordcito]

@yhirose proxy test was broken in another PR.

[yhirose]

@Dwordcito, thanks for the pull request. However, I am not going to merge it because this modification affects a request handler with ContentReceiver. In this case, res.body will not be used at all, but this change allocates unnecessary memory.

  svr.Post("/content_receiver", [&](const Request &, Response &res,
                                    const ContentReader &content_reader) {
    content_reader([&](const char *data, size_t data_length) {
      // Handle `data` without pushing it to `res.body` at all.
      return true;
    });
  });