I write an http request using curl like this:
```
curl_global_init(CURL_GLOBAL_ALL);
CURL* easy_handle = curl_easy_init();
curl_easy_setopt(easy_handle, CURLOPT_URL, "https://nghttp2.org/";);
curl_easy_setopt(easy_handle, CURLOPT_WRITEFUNCTION, WriteMemoryCallback);
data_chunk_t chunk;
chunk.data = malloc(1); /* will be grown as needed by the realloc above */
chunk.size = 0; /* no data at this point */
curl_easy_setopt(easy_handle, CURLOPT_WRITEDATA, &chunk);
curl_easy_setopt(easy_handle, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_2_0);
curl_easy_setopt(easy_handle, CURLOPT_SSL_VERIFYPEER, 0L);//忽略证书检查
curl_easy_setopt(easy_handle, CURLOPT_SSL_VERIFYHOST, 0L);
curl_easy_setopt(easy_handle, CURLOPT_PIPEWAIT, 1L);
curl_easy_setopt(easy_handle, CURLOPT_VERBOSE, 1L);
curl_easy_setopt(easy_handle, CURLOPT_DEBUGFUNCTION, my_trace);
CURLcode code;
code = curl_easy_perform(easy_handle);
long retcode = 0;
code = curl_easy_getinfo(easy_handle, CURLINFO_RESPONSE_CODE, &retcode);
```
but the debug call back showed me that the request header is
```
GET / HTTP/1.1
Host: nghttp2.org
Accept: */*
```
it's not an http2 request, why? What should I do?
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