Wrong order when ordering by StringLast agg with GroupBy V2

### Affected Version

0.13.0+

### Description

A user on the mailing list reported the following issue: https://groups.google.com/d/msg/druid-user/x0xuqSFM794/C8tn3J4eDAAJ

The following data, index task, and queries can be used to reproduce the described issue:

data
```
{"time":"2015-09-12T00:46:58.771Z", "personId":"1111", "name":"ZZZZZ", "age": 30}
{"time":"2015-09-12T01:46:58.771Z", "personId":"1111", "name":"BBBBB", "age": 31}
{"time":"2015-09-12T02:46:58.771Z", "personId":"2222", "name":"YYYYY", "age": 20}
{"time":"2015-09-12T03:46:58.771Z", "personId":"2222", "name":"AAAAA", "age": 21}
```

task
```
{
"type" : "index",
"spec" : {
"dataSchema" : {
"dataSource" : "stringlaster",
"parser" : {
"type" : "string",
"parseSpec" : {
"format" : "json",
"dimensionsSpec" : {
"dimensions" : [
"time",
"personId",
"name"
]
},
"timestampSpec" : {
"format" : "auto",
"column" : "time"
}
}
},
"metricsSpec" : [
{ "type" : "longSum", "name" : "age", "fieldName" : "age" }
],
"granularitySpec" : {
"type" : "uniform",
"segmentGranularity" : "MONTH",
"queryGranularity" : "none",
"intervals" : ["2015-09-01/2015-10-01"],
"rollup" : false
}
},
"ioConfig" : {
"type" : "index",
"firehose" : {
"type" : "local",
"baseDir" : "quickstart/",
"filter" : "stringlast_data.json"
},
"appendToExisting" : false
},
"tuningConfig" : {
"type" : "index",
"targetPartitionSize" : null,
"maxRowsInMemory" : 25000,
"forceGuaranteedRollup" : true,
"numShards": 2
}
}
}
```

query, ordering by string last (wrong results):
```
{
"queryType": "groupBy",
"dataSource": { "type": "table", "name": "stringlaster" },
"intervals": { "type": "intervals", "intervals": [ "2015-09-01/2015-10-01" ] },
"granularity": { "type": "all" },
"dimensions": [ "personId" ],
"aggregations": [
{ "type": "stringLast", "name": "name", "fieldName": "name" },
{ "type": "longLast", "name": "age", "fieldName": "age" }
],
"limitSpec": {
"type": "default", "limit": 1000,
"columns" : [{ "dimension":"name","direction":"ascending","dimensionOrder":"alphanumeric"}]
},
"context": {}
}
```

wrong results for the above query:
```
[
{
"version": "v1",
"timestamp": "2015-09-01T00:00:00.000Z",
"event": {
"name": "BBBBB",
"personId": "1111",
"age": 31
}
},
{
"version": "v1",
"timestamp": "2015-09-01T00:00:00.000Z",
"event": {
"name": "AAAAA",
"personId": "2222",
"age": 21
}
}
]
```

query ordering by long last (correct results)
```
{
"queryType": "groupBy",
"dataSource": { "type": "table", "name": "stringlaster" },
"intervals": { "type": "intervals", "intervals": [ "2015-09-01/2015-10-01" ] },
"granularity": { "type": "all" },
"dimensions": [ "personId" ],
"aggregations": [
{ "type": "stringLast", "name": "name", "fieldName": "name" },
{ "type": "longLast", "name": "age", "fieldName": "age" }
],
"limitSpec": {
"type": "default", "limit": 1000,
"columns" : [{ "dimension":"age","direction":"ascending","dimensionOrder":"alphanumeric"}]
},
"context": {}
}
```

right results for above query:
```
[
{
"version": "v1",
"timestamp": "2015-09-01T00:00:00.000Z",
"event": {
"name": "AAAAA",
"personId": "2222",
"age": 21
}
},
{
"version": "v1",
"timestamp": "2015-09-01T00:00:00.000Z",
"event": {
"name": "BBBBB",
"personId": "1111",
"age": 31
}
}
]
```

Use the supplied indexing task and GroupBy V2 queries to reproduce the StringLast ordering result, then compare it with the LongLast ordering case. Done means ordering by the stringLast aggregation returns the expected personId order and values shown in the issue.

Written by the indexing model from the issue text.