evhttp returns text/plain only

http_plain.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <event.h>
#include <evhttp.h>

#define HTTPD_ADDR "0.0.0.0"
#define HTTPD_PORT 8080

void req_handler(struct evhttp_request *r, void *arg)
{
  struct evbuffer *evbuf;
  char message[] = "Hello World";
  int message_length = strlen(message);
  char content_length[8];

  memset(content_length, 0, 8);
  snprintf(content_length, 7, "%d", message_length);

  evhttp_add_header(r->output_headers, "Content-Type", "text/plain");
  evhttp_add_header(r->output_headers, "Content-Length", content_length);

  evbuf = evbuffer_new();
  if (evbuf == NULL) {
    evhttp_send_error(r, HTTP_SERVUNAVAIL, "failed to create buffer");
    return;
  }
  evbuffer_add(evbuf, message, message_length);
  evhttp_send_reply(r, HTTP_OK, "", evbuf);
  evbuffer_free(evbuf);
}

int main(int argc, const char* argv[])
{
  struct event_base *ev_base;
  struct evhttp *httpd;

  ev_base = event_base_new();

  httpd = evhttp_new(ev_base);
  if (evhttp_bind_socket(httpd, HTTPD_ADDR, HTTPD_PORT) < 0) {
    perror("evhttp_bind_socket()");
    exit(EXIT_FAILURE);
  }
  evhttp_set_gencb(httpd, req_handler, NULL);

  event_base_dispatch(ev_base);

  evhttp_free(httpd);
  event_base_free(ev_base);

  return 0;
}