(Assigning this to Martin)

I really like this patch, and I support that we land this before the feature freeze of 3.6. 

As along as the API interface to users is simple, the changes internal to _send_output should not be a concern. Yeah, I agree that multiple things needs to be taken care, but it is alright.

Also, +1 to do validation of transfer-encoding usage as it's done by the patch. This is along the lines of RFC recommendation and it is better to fail early than let the server implementation be the decider. 

I would like to know the better suggestion for "testing text" too. (Adding haypo to steer us here) Although, I would point out that the static method has a generic name _is_textIO, while relying on .read attribute which is checked outside the static method.