Sets the headers to prevent caching for the different browsers.

Different browsers support different nocache headers, so several
headers must be sent so that all of them get the point that no
caching should occur.

function nocache_headers() {
	if ( headers_sent() ) {
		return;
	}

	$headers = wp_get_nocache_headers();

	unset( $headers['Last-Modified'] );

	header_remove( 'Last-Modified' );

	foreach ( $headers as $name => $field_value ) {
		header( "{$name}: {$field_value}" );
	}
}