allow set passwords_hashed option for digest auth

[lHydra]

Rack Digest Auth allows to check the user's credentials with ha1 digest hash instead of check with a plain password. But at the moment this functionality is not available because the condition from Rack::Auth::Digest::MD5 is always false

if opaque.nil? and realm.respond_to? :values_at
  realm, opaque, @passwords_hashed = realm.values_at :realm, :opaque, :passwords_hashed
end

(in the grape auth dsl we constantly set the option opaque - options[:opaque] ||= 'secret')

[dblock]

Needs a rubocop -a, sorry ;)

[lHydra]

Needs a rubocop -a, sorry ;)

fixed

[dblock]

Typos in README, sorry didn't catch them earlier.