RFC-compatible ETag/Header processing

Make ETag handling RFC-compliant.
See:
* http://tools.ietf.org/html/rfc2616#section-4.2
* http://www.w3.org/Protocols/rfc2616/rfc2616-sec2.html#sec2.2
* http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.11
* https://stackoverflow.com/q/7886782

If using perl as dependency in `supload` is not an option for you this could be tenchinally be done with awk (already used in script) but I don't know how to make it properly handling non-printable chars in regexps.

Or at least consider to use following awk one-liner to handle ETag which is much simpler than the current monstrous pipeline:

`awk -vIGNORECASE=1 '/^etag:/{print $2}'`