fix address extraction from anonymous set

[myromeu]

Fixes issue #22
Also take account about mixed sets with plain address and subnets like this:

{
  "match": {
    "op": "==",
    "left": {
      "payload": {
        "protocol": "ip",
        "field": "saddr"
      }
    },
    "right": {
      "set": [
        "8.8.8.8",
        "10.96.0.10",
        "21.21.0.242",
        {
          "prefix": {
            "addr": "127.0.0.0",
            "len": 8
          }
        }
      ]
    }
  }
}

[majst01]

Thanks for your PR, can you by any chance add some unit-tests to this new functionality ?

[myromeu]

@majst01 yes, I will do it little bit latter (mb in mid next week)

[myromeu]

@majst01 hello)

I've finished with tests. Can you check and merge?