[Do not merge] MGET endpoint

experiencing problems with the backend client. 

ideally, the cmd thats sent to the elasticsearch client should look like this 

```
[ 
  { index: 'pelias', type: 'geoname', id: '4193605' },
  { index: 'pelias', type: 'geoname', id: '4193595' }
]
```

but the backend errors out with the following error message: 
```
{ [Error: ElasticsearchParseException[Failed to derive xcontent from org.elasticsearch.common.bytes.BytesArray@1]]
  message: 'ElasticsearchParseException[Failed to derive xcontent from org.elasticsearch.common.bytes.BytesArray@1]' }
```

Then, I looked at https://github.com/geopipes/elasticsearch-backend/blob/master/index.js#L38 and changed my cmd to just be an array of ids
```
{ 
  index: 'pelias',
  type: 'geoname',
  ids: [ '4193605', '4193595' ] 
}
```
but it errored out with the following
```
{ [Error: ElasticsearchParseException[Failed to derive xcontent from org.elasticsearch.common.bytes.ChannelBufferBytesReference@efc39159]]
  message: 'ElasticsearchParseException[Failed to derive xcontent from org.elasticsearch.common.bytes.ChannelBufferBytesReference@efc39159]' }
```

I might have to rewrite the mget endpoint in the elasticsearch-backend, right? Or am I missing something? I say rewrite because if sets one type for multiple ids but the reality is that we might have a batch request with different types (geoname/123, osmnode/1234 etc)

Thoughts?