Aggregates are wrong for non unique columns

[jamescasbon]

If you have a repeated value in a Data.Collection, the aggregation produces incorrect values:

Test data, note we have two german countries with the same population:

var countries_data = {
"properties": {
"name": {"name": "Country Name", "type": "string", "unique": true },
"official_language": {"name": "Official Language", "type": "string", "unique": false },
"population": { "name": "Population", "type": "number", "unique": false }
},
"items": {
0: {
"name": "Austria",
"official_language": "German",
"population": 8356700,
},

  'usa': {
    "name": "United States of America",
    "official_language": "English",
    "population": 310955497,
  },
  'ger': {
    "name": "Germany",
    "official_language": "German",
    "population": 82062200,
  },
  'ger2': {
    "name": "Germany2",
    "official_language": "German",
    "population": 82062200,
  }
}

}

var countries = new Data.Collection(countries_data);
console.log('oh sweet, just made a collection:');
console.log(countries_data);

var population = countries.properties().get('population');
console.log('get returns a Data.Property: ');
console.log(population);

console.log('which supports aggregation: ');
var population_sum = population.aggregate(Data.Aggregators.SUM) // => Returns total population of all countries in the collection.
console.log(population_sum);

equals(population_sum, 310955497 + 82062200 + 8356700 + 82062200, 'We can aggregate a column');

This test fails, I think because Data.Property.aggregate is defined:

aggregate: function (fn) {
  return fn(this.values("values"));
},

this.values produces a hash where each key is the value considered, so in this case it has three rather than 4 values do to the repeated value.