fix assert_select_email to work on non-multipart emails as well as converting...

fix assert_select_email to work on non-multipart emails as well as converting the Mail::Body to a string to prevent errors.

actionpack/lib/action_dispatch/testing/assertions/selector.rb

@@ -415,9 +415,9 @@ def assert_select_email(&block)
         assert !deliveries.empty?, "No e-mail in delivery list"
 
         for delivery in deliveries
-          for part in delivery.parts
+          for part in (delivery.parts.empty? ? [delivery] : delivery.parts)
             if part["Content-Type"].to_s =~ /^text\/html\W/
-              root = HTML::Document.new(part.body).root
+              root = HTML::Document.new(part.body.to_s).root
               assert_select root, ":root", &block
             end
           end

actionpack/test/controller/assert_select_test.rb

@@ -20,6 +20,15 @@ def test(html)
     end
   end
 
+  class AssertMultipartSelectMailer < ActionMailer::Base
+    def test(options)
+      mail :subject => "Test e-mail", :from => "test@test.host", :to => "test <test@test.host>" do |format|
+        format.text { render :text => options[:text] }
+        format.html { render :text => options[:html] }
+      end
+    end
+  end
+
   class AssertSelectController < ActionController::Base
     def response_with=(content)
       @content = content
@@ -313,6 +322,16 @@ def test_assert_select_email
     end
   end
 
+  def test_assert_select_email_multipart
+    AssertMultipartSelectMailer.test(:html => "<div><p>foo</p><p>bar</p></div>", :text => 'foo bar').deliver
+    assert_select_email do
+      assert_select "div:root" do
+        assert_select "p:first-child", "foo"
+        assert_select "p:last-child", "bar"
+      end
+    end
+  end
+
   protected
     def render_html(html)
       @controller.response_with = html