content_encoding: Add "none" alias to "identity"

Some servers return a "content-encoding" header with a non-standard
"none" value.

Add "none" as an alias to "identity" as a work-around, to avoid
unrecognised content encoding type errors.

Signed-off-by: Mohammad AlSaleh <CE.Mohammad.AlSaleh@gmail.com>

Closes https://github.com/curl/curl/pull/2298