scroll REST API should support source parameter

As stated in documentation, we should support `?source=` parameter in `/_search/scroll` REST operations.

This is how to reproduce it:

```sh
curl -XDELETE "http://localhost:9200/test"
curl -XPOST "http://localhost:9200/test/type/1" -d'
{
    "foo": "bar"
}'

# This one works
curl -XPOST "http://localhost:9200/_search/scroll" -d "FAKESCROLLID"

# This one gives: {"error":"Failed to derive xcontent from org.elasticsearch.common.bytes.BytesArray@0"}
curl -XGET "http://localhost:9200/_search/scroll/?source=FAKESCROLLID"
```

Closes #4941.