Fixed bug on HTML only emails getting set to text/plain

actionmailer/lib/action_mailer/base.rb

@@ -531,7 +531,7 @@ module ActionMailer #:nodoc:
       when m.multipart?
         ["multipart", "alternative", params]
       else
-        class_default
+        m.content_type || class_default
       end
     end
 

actionmailer/test/base_test.rb

@@ -17,6 +17,14 @@ class BaseTest < ActiveSupport::TestCase
     def simple(hash = {})
       mail(hash)
     end
+
+    def html_only(hash = {})
+      mail(hash)
+    end
+
+    def plain_text_only(hash = {})
+      mail(hash)
+    end
     
     def simple_with_headers(hash = {})
       headers hash
@@ -434,6 +442,16 @@ class BaseTest < ActiveSupport::TestCase
     mail = BaseMailer.explicit_multipart
     assert_not_nil(mail.content_type_parameters[:boundary])
   end
+  
+  test "should set a content type if only has an html part" do
+    mail = BaseMailer.html_only
+    assert_equal('text/html', mail.mime_type)
+  end
+  
+  test "should set a content type if only has an plain text part" do
+    mail = BaseMailer.plain_text_only
+    assert_equal('text/plain', mail.mime_type)
+  end
 
   protected
 

actionmailer/test/fixtures/base_mailer/html_only.html.erb

@@ -0,0 +1 @@
+<h1>Testing</h1>

actionmailer/test/fixtures/base_mailer/plain_text_only.text.erb

@@ -0,0 +1 @@
+Testing