add failing test for broadcast

[roadscape]

Error: Error Response: {'code': -32000, 'message': 'Bad Cast:Invalid cast from string_type to Array', 'data': {'code': 7, 'name': 'bad_cast_exception', 'message': 'Bad Cast', 'stack': [{'context': {'level': 'error', 'file': 'variant.cpp', 'line': 537, 'method': 'get_array', 'hostname': '', 'timestamp': '2018-03-27T20:28:38'}, 'format': 'Invalid cast from ${type} to Array', 'data': {'type': 'string_type'}}, {'context': {'level': 'warn', 'file': 'asset.cpp', 'line': 409, 'method': 'from_variant', 'hostname': '', 'timestamp': '2018-03-27T20:28:38'}, 'format': '', 'data': {}}]}}

Body: {"jsonrpc": "2.0", "id": 0, "method": "call", "params": ["database_api", "verify_authority", [{"ref_block_num": 13984, "ref_block_prefix": 551221541, "expiration": "2018-03-27T20:32:10", "operations": [["claim_reward_balance", {"account": "test", "reward_steem": "0.000 STEEM", "reward_sbd": "0.000 SBD", "reward_vests": "0.000000 VESTS"}]], "extensions": [], "signatures": ["1f2ee28e71e107a5c8a78bd5a53439ec26001da44ce81370b53f6f29a305012d6246162c796000e596701bbe02ec52e47ffdfe05e815a076d983ac98ef26b9e277"]}]]}

Body-pp:

{
  "jsonrpc": "2.0",
  "id": 0,
  "method": "call",
  "params": [
    "database_api",
    "verify_authority",
    [
      {
        "ref_block_num": 13984,
        "ref_block_prefix": 551221541,
        "expiration": "2018-03-27T20:32:10",
        "operations": [
          [
            "claim_reward_balance",
            {
              "account": "test",
              "reward_steem": "0.000 STEEM",
              "reward_sbd": "0.000 SBD",
              "reward_vests": "0.000000 VESTS"
            }
          ]
        ],
        "extensions": [],
        "signatures": [
          "1f2ee28e71e107a5c8a78bd5a53439ec26001da44ce81370b53f6f29a305012d6246162c796000e596701bbe02ec52e47ffdfe05e815a076d983ac98ef26b9e277"
        ]
      }
    ]
  ]
}

[roadscape]

Two new tests: test_claim_reward, test_witness_update -- both are now passing against pre-appbase nodes and failing against appbase during verify_authority call, tracked at https://github.com/steemit/steem/issues/2284