Also inject analytics with headers, not just in footer

The current before_footer hook works great for Google Analytics,
but isn't 100% consistent for Piwik as usually the page has
often rendered the header by that time.

Change-Id: Ifac6917818e33db022e1a5522a58b229a9e3e369

lib.php

@@ -37,3 +37,11 @@
 function local_analytics_before_footer() {
     injector::inject();
 }
+
+/**
+ * Output callback, available since Moodle 3.3
+ *
+ */
+function local_analytics_before_http_headers() {
+    injector::inject();
+}