Fix key equality comparison in LookupTable map (#1379)

Fix key equality comparison in LookupTable map (#1379)

An std::unordered_map with case-insensitive keys must use a
case-insensitive key equality comparison. lookupTable_t used (the
default) std::equal_to<SBuf> comparison which is case-sensitive.

The full extent of this bug effects is unknown, but Squid was
mishandling Cache-Control and Surrogate-Control directives with
non-canonical (i.e. other than all-lower-case) spelling. Similar
problems affected WWW-Authenticate Digest parameter names, but the
related code remains inconsistent, with both case-sensitive and
case-insensitive checks applied to some of the key names in
Auth::Digest::Config::decode().

Also removed a similarly buggy (and, technically, unused) "typical use"
example from the CaseInsensitiveSBufHash class description. C++
documentation and Squid code are better sources of usage examples when
it comes to STL-used concepts like hash function objects.

This minimal fix excludes LookupTable class polishing.

The bug was introduced in 2015 commit 81ab22b.