Add transfer_encoding= setter deprecation.

actionmailer/lib/action_mailer/tmail_compat.rb

@@ -17,7 +17,13 @@ def transfer_encoding(value = nil)
         old_transfer_encoding
       end
     end
-    
+
+    def transfer_encoding=(value)
+      ActiveSupport::Deprecation.warn('Message#transfer_encoding= is deprecated, please call ' <<
+                                      'Message#content_transfer_encoding= with the same arguments', caller[0,2])
+      self.content_transfer_encoding = value
+    end
+
     def original_filename
       ActiveSupport::Deprecation.warn('Message#original_filename is deprecated, ' <<
                                       'please call Message#filename', caller[0,2])

actionmailer/test/old_base/tmail_compat_test.rb

@@ -21,5 +21,15 @@ def test_transfer_encoding_raises_deprecation_warning
     end
     assert_equal mail.content_transfer_encoding, "base64"
   end
+
+  def test_transfer_encoding_setter_raises_deprecation_warning
+    mail = Mail.new
+    assert_deprecated do
+      assert_nothing_raised do
+        mail.transfer_encoding = "base64"
+      end
+    end
+    assert_equal mail.content_transfer_encoding, "base64"
+  end
   
 end