assert_template changed behaviour from 3.x to 4.0.x

Hi.

I have a situation where the user is POSTing to the controller and the
controller will
1) send an email using a view template
2) respond xml over HTTP response without using a view template

The test contains a line with
assert_template ''

which verifies that no template is used for HTTP response as it simple
responds the object serialized as XML.

In 3.x this worked fine, but in 4.0.x the functionality (the
controller behaviour) still seems to work fine, that is it
1) send an email using a view template
2) respond xml over HTTP response without using a view template

but the test fails since the `assert_template ''` fails with a text like:
expecting <""> but rendering with <["notifier/my_email_view"]>

The notifier/my_email_view is the view I am using to send emails with.

So the assert_template catches the view used to send email and I am
trying to assert that no view is used to respond HTML. How can I
achieve that?

Any help is appreciated.

Jarl