Make _all field accessible with GET

GET only returned null even when stored if requested with GET like this:
    
`curl -XGET "http://localhost:9200/test/test/1?fields=_all"`
    
Instead, it should simply behave like a String field and return the concatenated fields as String.

You could use

```java
index("my-index", "my-type1", "1", "some_field", "some text");
```