Load 'redirect' module on 'send_headers' instead of 'plugins_loaded'.

Fixes an issue with checking for REST API requests since
'plugins_loaded' runs before the REST API is loaded.

See humanmade/Mercator-Redirect#11

redirect.php

@@ -4,7 +4,7 @@
 
 use Mercator\Mapping;
 
-add_action( 'plugins_loaded', __NAMESPACE__ . '\handle_redirect' );
+add_action( 'send_headers', __NAMESPACE__ . '\handle_redirect' );
 
 function is_enabled( $mapping = null ) {
 	$mapping = $mapping ?: $GLOBALS['mercator_current_mapping'];