mget REST API should support source parameter

As stated in documentation, we should support `?source=` parameter in mget REST operations.

This is how to reproduce it:

```sh
curl -XDELETE "http://localhost:9200/test"

curl -XPOST "http://localhost:9200/test/type/1?refresh" -d'{
    "foo": "bar"
}'

# This one works
curl -XPOST "http://localhost:9200/test/type/_mget" -d'{
    "ids": ["1"]
}'

# This one gives: {"error":"Failed to derive xcontent from org.elasticsearch.common.bytes.BytesArray@0"}
curl -XGET "http://localhost:9200/test/type/_mget?source=%7B%22ids%22%3A%20%5B%221%22%5D%7D"
```

Closes #4892.