Fix HF26 breaking change for beneficiaries in Streamer.js

[primersion]

In HF26 the form beneficiaries are handled has changed (breaking change), which results in hive-engine nodes diverging, due to the beneficiaires not being parsed correctly.
Before the beneficiary format was the following:

"extensions": [
    [
      0,
      {
        "beneficiaries": [
          {
            "account": "alinares",
            "weight": 300
          },
          {
            "account": "hiveonboard",
            "weight": 100
          },
          {
            "account": "tipu",
            "weight": 100
          }]
      }]
  ]
}]
],

and after HF26:

"extensions": [
    {
      "type": "comment_payout_beneficiaries",
      "value": {
        "beneficiaries": [
          {
            "account": "alinares",
            "weight": 300
          },
          {
            "account": "hiveonboard",
            "weight": 100
          },
          {
            "account": "tipu",
            "weight": 100
          }]
      }
    }]
}]
],

The change adds an additional if case parsing the new beneficiary format.

[primersion]

I have been running the change against a HF26 node for the past 12 hours, all without issues and the node is still in sync, so the change should be all good in my opinion.

[bt-cryptomancer]

Changes look good.