text-html type is absent in TXT_ENC

If we get response with 404 status code and this response
contains in headers "content-type: text/html", NotFound
exception isn't raised.
Add 'text/html' type of content and convert value of
this parameter to lower case.

Change-Id: I632701f43219fa6c0efc6e95bf6007cb0a3ff0c2
Closes-Bug: #1269712