Don't accept type wrapper in indexing request

Currently it is possible to index a document as:

```
POST /myindex/mytype/1
{ "foo"...}
```

or as:

```
POST /myindex/mytype/1
{
    "mytype": {
        "foo"...
    }
}
```

This makes indexing non-deterministic and fields can be misinterpreted
as type names.

This changes makes Elasticsearch accept only the first form, ie without
the type wrapper.

Relates to #4484