Default to not accepting type wrapper in indexing requests

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 by default,
ie without the type wrapper. This can be changed by setting
`index.mapping.allow_type_wrapper` to `true`` when creating the index.

Closes #4484