Fix a login issue on WP-Engine hosting

WP-Engine requires additional parameters to exist on the URL for POST requests,
and it applies those using a filter on the site_url hook when using the
login_post scheme. So I've made sure to apply that filter when running in a
WP-Engine environment.

class.two-factor-core.php

@@ -318,6 +318,12 @@ public static function login_html( $user, $login_nonce, $redirect_to, $error_msg
 		$interim_login = isset( $_REQUEST['interim-login'] ); // WPCS: override ok.
 		$wp_login_url = wp_login_url();
 
+		// If we're running in a WP-Engine environment then we need to apply the
+		// site_url filter with the login_post scheme for the form action.
+		if (function_exists('is_wpe') && is_wpe()) {
+			$wp_login_url = apply_filters( 'site_url', $wp_login_url, null, 'login_post', null);
+		}
+
 		$rememberme = 0;
 		if ( isset( $_REQUEST['rememberme'] ) && $_REQUEST['rememberme'] ) {
 			$rememberme = 1;