Error 500 during plugin activation on a default Apache2 installation

Hi,

On a default Apache2 installation the plugin breaks the site with a 500 error with the following details:

Invalid command 'Header', perhaps misspelled or defined by a module not included in the server configuration

That’s caused by the following line added to the .htaccess file by WP-Optimize:

Header append Vary User-Agent

The above line assumes that mod_headers is going to be available for any Apache2 installation, and therefore causing the error 500 if mod_headers is not available. This module is not enabled by default.

Using IfModule before the above line, as WP-Optimize does for other Apache2 modules, solves the issue:

<IfModule mod_headers.c>
	Header append Vary User-Agent
</IfModule>

A better alternative could be to remove that from the .htaccess and let WordPress take care of adding the header when the server allows it:

function wpo_add_vary_header( $headers ) {
	$headers['Vary'] = 'User-Agent';
	return $headers;
}
add_filter('wp_headers', 'wpo_add_vary_header');

Would be great to see any of the above fixes added to the plugin.