fix(aggr): field unmatched when fid duplicated

[k6sdevbob]

The method channelAggregate() used to search the field (referenced by the key "field" in encoding) in an array so that if there're more than one pills of a same field, e.g.:

[
    { "fid": "foo", "analyticType": "dimension" },
    { "fid": "foo", "analyticType": "measure", "aggName": "sum" }
]

In the case above, any pills with fid "foo" would match the first IViewField, which will make the aggregation ignored, though it's named "sum(foo)".

There's a real case -

The data in the vega-lite config looks like -

{
  "data": {
    "values": [
      {"gwc_3": 0, "gwc_3_sum": 0},
      {"gwc_3": 1, "gwc_3_sum": 724},
      {"gwc_3": 2, "gwc_3_sum": 1430},
      // ...
      {"gwc_3": 22, "gwc_3_sum": 16016},
      {"gwc_3": 23, "gwc_3_sum": 16744}
    ]
  }
}

Which is expected to be -

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[ObservedObserver]

need refactor