fix bug that was setting arrays as keys in default_options hash

Lines 26-28 in `lib/rack/ssl-enforcer.rb` are causing the `default_options` hash to look like this:

    {
      :redirect_to => nil,
      :redirect_code => nil,
      :strict => false,
      :mixed => false,
      :hsts => nil,
      :http_port => nil,
      :https_port => nil,
      :force_secure_cookies => true,
      [:only_hosts, :except_hosts] => nil,
      [:only, :except] => nil,
      [:only_methods, :except_methods] => nil
    }

This fixes it to look like this:

    {
      :redirect_to => nil,
      :redirect_code => nil,
      :strict => false,
      :mixed => false,
      :hsts => nil,
      :http_port => nil,
      :https_port => nil,
      :force_secure_cookies => true,
      :only_hosts => nil,
      :except_hosts => nil,
      :only => nil,
      :except => nil,
      :only_methods => nil,
      :except_methods => nil
    }