--reap flag is not added to iptables command

In lib/puppet/provider/firewall/iptables.rb we test on boolean flags when building iptables args:

        # If socket is true then do not add the value as -m socket is standalone
        if known_booleans.include?(res) then
          if resource[res] == :true then
            resource_value = nil
          else
            # If the property is not :true then we don't want to add the value
            # to the args list
            next
          end
        end

This evaluates to false on the reap flag in a definition like this:
    firewall { '001 rate limit ssh attempts':
        port   => [22],
        proto  => tcp,
        tcp_flags => "FIN,SYN,RST,ACK SYN",
        recent => 'rcheck',
        rsource => true,
        rname => 'ssh-syn4',
        rseconds => 30,
        rhitcount => 3,
        reap => true,
        jump => drop,
    }

This is because the value is not defined as a string, so the reap flag is not added to the args. This patch defines reap as a string true or false to match others like rsource.