Fix for emails with empty body

lib/email/message_builder.rb

@@ -7,7 +7,9 @@ module Email
       builder = Email::MessageBuilder.new(*builder_args)
       headers(builder.header_args) if builder.header_args.present?
       mail(builder.build_args).tap { |message|
-        message.html_part = builder.html_part if message
+        if message and h = builder.html_part
+          message.html_part = h
+        end
       }
     end
   end