time_zone should set pre AND post _zone in aggs

Until now, with aggregations and facets, when users use `time_zone` we get back some inaccurate results.

The cause is that we set only `pre_zone` when `time_zone` is set. So after computing values, we don't apply `post_zone` to results, which gives "strange" results.

Here is a full recreation:

Create the test
------------

```
DELETE /test

PUT /test/doc/1
{
  "date" : "2014-04-25T21:05:00"
}

PUT /test/doc/2
{
  "date" : "2014-04-25T22:06:00"
}

PUT /test/doc/3
{
  "date" : "2014-04-25T23:07:00"
}

PUT /test/doc/4
{
  "date" : "2014-04-26T00:08:00"
}
```

Group by date
-----------

```
GET /test/doc/_search
{
  "size": 0,
  "aggs": {
    "bydate": {
      "date_histogram": {
        "field": "date",
        "interval": "day",
        "time_zone": "Europe/Paris"
      }
    }
  }
}
```

This gives

```
   "aggregations": {
      "bydate": {
         "buckets": [
            {
               "key_as_string": "2014-04-25T00:00:00.000Z",
               "key": 1398384000000,
               "doc_count": 1
            },
            {
               "key_as_string": "2014-04-26T00:00:00.000Z",
               "key": 1398470400000,
               "doc_count": 3
            }
         ]
      }
   }
```

When `pre_zone` and `post_zone` are used:

```
GET /test/doc/_search
{
  "size": 0,
  "aggs": {
    "bydate": {
      "date_histogram": {
        "field": "date",
        "interval": "day",
        "pre_zone": "Europe/Paris",
        "post_zone": "Europe/Paris"
      }
    }
  }
}
```

We get:

```
   "aggregations": {
      "bydate": {
         "buckets": [
            {
               "key_as_string": "2014-04-25T02:00:00.000Z",
               "key": 1398391200000,
               "doc_count": 1
            },
            {
               "key_as_string": "2014-04-26T02:00:00.000Z",
               "key": 1398477600000,
               "doc_count": 3
            }
         ]
      }
   }
```

Grouping by hours
--------------

It's even more obvious when grouping by hours:

```
GET /test/doc/_search
{
  "size": 0,
  "aggs": {
    "bydate": {
      "date_histogram": {
        "field": "date",
        "interval": "hour",
        "time_zone": "Europe/Paris"
      }
    }
  }
}
```

gives

```
   "aggregations": {
      "bydate": {
         "buckets": [
            {
               "key_as_string": "2014-04-25T21:00:00.000Z",
               "key": 1398459600000,
               "doc_count": 1
            },
            {
               "key_as_string": "2014-04-25T22:00:00.000Z",
               "key": 1398463200000,
               "doc_count": 1
            },
            {
               "key_as_string": "2014-04-25T23:00:00.000Z",
               "key": 1398466800000,
               "doc_count": 1
            },
            {
               "key_as_string": "2014-04-26T00:00:00.000Z",
               "key": 1398470400000,
               "doc_count": 1
            }
         ]
      }
   }
```

and

```
GET /test/doc/_search
{
  "size": 0,
  "aggs": {
    "bydate": {
      "date_histogram": {
        "field": "date",
        "interval": "hour",
        "pre_zone": "Europe/Paris",
        "post_zone": "Europe/Paris"
      }
    }
  }
}

```

gives

```
   "aggregations": {
      "bydate": {
         "buckets": [
            {
               "key_as_string": "2014-04-25T23:00:00.000Z",
               "key": 1398466800000,
               "doc_count": 1
            },
            {
               "key_as_string": "2014-04-26T00:00:00.000Z",
               "key": 1398470400000,
               "doc_count": 1
            },
            {
               "key_as_string": "2014-04-26T01:00:00.000Z",
               "key": 1398474000000,
               "doc_count": 1
            },
            {
               "key_as_string": "2014-04-26T02:00:00.000Z",
               "key": 1398477600000,
               "doc_count": 1
            }
         ]
      }
   }
```

This patch fix this.