digest: unquote realm and nonce before processing

digest: unquote realm and nonce before processing

RFC 7616 (and 2617) requires values to be "unquoted" before used for
digest calculations. The only place where unquoting can be done
correctly is header parsing function (realm="DOMAIN\\host" and
realm=DOMAN\\host are different realms).

This commit adds unquoting (de-escaping) of all values during header
parsing and quoting of the values during header forming. This approach
should be most straightforward and easy to read/maintain as all values
are processed in the same way as required by RFC.

Closes #8912