[CTG] Alteration of header fields (4.1.5) / inconsistencies

> - any other header that could be changed by a "regular" 
> proxy?

In the context of requests, at least the following header fields might undergo modifications: Content-Length, Content-Encoding, and Warning. 

Scenario: From RFC2616 14.11, a non-transparent proxy may change the encoding of the body. In a POST request, this implies that the Content-Length and Content-Encoding must be adjusted accordingly, and a Warning header field must be inserted or extended.

In the context of CT-proxies, it is easy to envision further modifications of the request body that would entail modification of the aforementioned header fields. 

Which means that the proposed resolution:

> "Proxies SHOULD NOT change headers other than
> User-Agent and Accept(-*) headers, and 
> X-Forwarded-For and Via as noted under 4.1.6
> Additional HTTP Headers if already present, and MUST
> NOT delete headers.

leads to a further inconsistency, as per the scenario above...

E.Casais