XML-RPC: Set HTTP status code in accordance with the spec.

When the XML-RPC endpoint is enabled, always return a HTTP `200 OK` status code in accordance with the XML-RPC specification. Continue to return an HTTP `405 Method Not Allowed` status code when the endpoint is disabled.

Props ariskataoka, johnbillion.
Fixes #52958.


Built from https://develop.svn.wordpress.org/trunk@50954


git-svn-id: http://core.svn.wordpress.org/trunk@50563 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/IXR/class-IXR-server.php

@@ -130,10 +130,6 @@ EOD;
             $error = new IXR_Error($error, $message);
         }
 
-		if ( function_exists( 'status_header' ) ) {
-			status_header( $error->code );
-		}
-
         $this->output($error->getXml());
     }