Send 'nosniff' header (fixes #97)

Change-Id: If9670a66a9402da77616a362ceda8ea47aceaa75
diff --git a/lib/Kalamar.pm b/lib/Kalamar.pm
index a6c136e..6a6a7cf 100644
--- a/lib/Kalamar.pm
+++ b/lib/Kalamar.pm
@@ -108,6 +108,12 @@
       });
   };
 
+  $self->hook(
+    before_dispatch => sub {
+      shift->res->headers->header('X-Content-Type-Options' => 'nosniff');
+    }
+  );
+
   $conf->{proxy_host} //= 1;
 
   # Take proxy host