Change response format of term vector endpoint

This commit changes the response format of the term vectors
to be consistent with the response format of the analyze endpoint.

```
{
   "_index": "test",
   "_type": "type1",
   "_id": "1",
   "_version": 1,
   "exists": true,
   "term_vectors": {
      "field_with_positions_offsets": {
         "field_statistics": {..},
         "terms": {
            "evil": {
               "term_freq": 2,
               "pos": [ 4 , 7 ],
               "start": [ 17, 40 ],
               "end": [ 21 , 44 ]
            },
            "orthodontist": {
               "term_freq": 1,
               "pos": [ 5 ]
               ],
               "start": [ 22 ],
               "end": [ 34]
            }
         }
      }
   }
}
```

becomes

```
{
   "_index": "test",
   "_type": "type1",
   "_id": "1",
   "_version": 1,
   "exists": true,
   "term_vectors": {
      "field_with_positions_offsets": {
         "field_statistics": {..},
         "terms": {
            "evil": {
               "term_freq": 2,
               "tokens": [
                   { "position": 4, "start_offset": 17, "end_offset" : 21 },
                   { "position": 7, "start_offset": 40, "end_offset" : 44 }
               ]
            },
            "orthodontist": {
               "term_freq": 1,
               "tokens" : [ { "position": 5 , "start_offset" : 22, "end_offset" : 34 } ]
            }
         }
      }
   }
}
```

Closes issue #3484