Following Twisted documentation, I found out that the warning method
doesn't exist on logger class. That class has warn method instead. And I
got a error because of that.

https://twistedmatrix.com/documents/16.4.1/api/twisted.logger.Logger.html

Before we used to have content_doc.content_type = 'text/plain;
charset="utf-8"'.
Currently, we have content_doc.content_type = 'text/plain'.

This change will give us, on top of that: content_doc.charset = 'utf-8'.
One can reconstruct the original one using .content_type and .charset.