Introduce HSTS header for https_only conf

Change-Id: If2aafcda5478cdfc91a65220542e420ca6893e07
diff --git a/lib/Kalamar.pm b/lib/Kalamar.pm
index 4a2ae7c..a6c136e 100644
--- a/lib/Kalamar.pm
+++ b/lib/Kalamar.pm
@@ -81,6 +81,13 @@
 
     # ... for cookie transport
     $self->sessions->secure(1);
+
+    # For all pages
+    $self->hook(
+      before_dispatch => sub {
+        shift->res->headers->header('Strict-Transport-Security' => 'max-age=3600; includeSubDomains');
+      }
+    );
   };
 
   # Run the app from a subdirectory