compression subsystem can fudge ETag header

When a Response object's $allow_compression_etag flag is true (on by default),
and when content-encoding compression is applied, and the Response has a
strong ETag, the ETag will be modified to include the compression method
as a prefix.  E.g:  `"deflate:8ae6b84:cd5d1e0d1f90fc1f61d68171feef4abf"`
version bump