nsIEncodedChannel in Firefox 3.0

 This attribute controls whether or not content conversion should be
 done per the Content-Encoding response header.  applyConversion can only 
 be set before or during OnStartRequest.  Calling this during 
 OnDataAvailable is an error. 

 TRUE by default.

 This attribute holds the MIME types corresponding to the content
 encodings on the channel.  The enumerator returns nsISupportsCString
 objects.  The first one corresponds to the outermost encoding on the
 channel and then we work our way inward.  "identity" is skipped and not
 represented on the list.  Unknown encodings make the enumeration stop.
 If you want the actual Content-Encoding value, use
 getResponseHeader("Content-Encoding").

 When there is no Content-Encoding header, this property is null.

 Modifying the Content-Encoding header on the channel will cause
 this enumerator to have undefined behavior.  Don't do it.

 Also note that contentEncodings only exist during or after OnStartRequest.
 Calling contentEncodings before OnStartRequest is an error.