Creating documents with an existing ID isn't possible

[Radiergummi]

Creating documents and setting their ID (eg. $doc->setAttribute('_id', 'foo')) is rejected as the ID is erroneously set on the request body in addition to the metadata field. This fails with the following error:

{
   "error": {
      "root_cause": [
         {
            "type": "mapper_parsing_exception",
            "reason": "Field [_id] is a metadata field and cannot be added inside a document. Use the index API request parameters."
         }
      ],
      "type": "mapper_parsing_exception",
      "reason": "failed to parse field [_id] of type [_id] in document with id 'foo'. Preview of field's value: 'foo'",
      "caused_by": {
         "type": "mapper_parsing_exception",
         "reason": "Field [_id] is a metadata field and cannot be added inside a document. Use the index API request parameters."
      }
   },
   "status": 400
}

We should ensure to remove any metadata attributes from the request body here.