wp-comments-post server protocol change

[kobashicomputing]

Hardcoded HTTP 1.1 is no longer valid anymore. Could be HTTP/2.

Change from this:

<?php
if ( 'POST' != $_SERVER['REQUEST_METHOD'] ) {
        header('Allow: POST');
        header('HTTP/1.1 405 Method Not Allowed');
        header('Content-Type: text/plain');
        exit;
}

To this:

<?php
if ('POST' != $_SERVER['REQUEST_METHOD']) {
  header('Allow: POST');
  header($_SERVER['SERVER_PROTOCOL'] . ' 405 Method Not Allowed');
  header('Content-Type: text/plain');
  exit;
}

[johnbillion]

Duplicate of #37863.