render :text with no layout fails when called from view or helper

In master:

<%= render :text => 'Hello world!' %>

produces no output. Likewise when you call render :text from a helper.

This example is contrived, but it seems like for consistency it should work.

My patch for #3260 ActionView::TestCase needs to be ported to master. includes a fix for this, but here's a new patch that isolates the fix and provides a test.