/*
 * call-seq:
 *   easy.content_type                                => "content/type" or nil
 *
 * Retrieve the content-type of the downloaded object. This is the value read
 * from the Content-Type: field. If you get +nil+, it means that the server
 * didn't send a valid Content-Type header or that the protocol used doesn't
 * support this.
 */
static VALUE ruby_curl_easy_content_type_get(VALUE self) {