inner_hits: Added another more compact syntax for inner hits.

Adds a more compact syntax that is sufficient most of the times:

```json
{
    "query" : {
        "nested" : {
            "path" : "comments",
            "query" : {
                "match" : {"comments.message" : "[actual query]"}
            },
            "inner_hits" : {}
        }
    }
}
```

which would otherwise look like:
```json
{
    "query" : {
        "nested" : {
            "path" : "comments",
            "query" : {
                "match" : {"comments.message" : "[actual query]"}
            }
        }
    },
    "inner_hits" : {
        "comments" : {
            "path" : { 
                "comments" : { 
                    "query" : {
                        "match" : {"comments.message" : "[actual query]"}
                    }
                }
            }
        }
    }
}
```
startsWith would also work, but containsKey is sufficient. We try to find the doc mapper that has an object mapper equal to path.