some part of digest auth header did not conform to RFC 2617

[RFC 2617](http://www.ietf.org/rfc/rfc2617.txt) says that qop-value and nc-value of the Authoriztion request should no be quoted. (in BNF).

I've fixed LWP::Authn::Digest to treat qop and nc as special case at the point of creating Authorization header. (and added test case for digest auth in local/http.t)

in fact, some servers (e.g. apache tomcat) doesn't accept quoted qop/nc-value.
[RT#43354](https://rt.cpan.org/Public/Bug/Display.html?id=43354) says almost same problems.