title: MongoSource from Kafka connect create weird _data key
body: <p>I'm using KafkaConnect - MongoSource with the following configuration:</p>
  
  <pre><code>curl -X PUT http://localhost:8083/connectors/mongo-source2/config -H "Content-Type: application/json" -d '{
    "name":"mongo-source2",
    "tasks.max":1,
    "connector.class":"com.mongodb.kafka.connect.MongoSourceConnector",
    "key.converter":"org.apache.kafka.connect.storage.StringConverter",
    "value.converter":"org.apache.kafka.connect.storage.StringConverter",
    "connection.uri":"mongodb://xxx:xxx@localhost:27017/mydb",
    "database":"mydb",
    "collection":"claimmappingrules.66667777-8888-9999-0000-666677770000",
    "pipeline":"[{\"$addFields\": {\"something\":\"xxxx\"} }]",
    "transforms":"dropTopicPrefix",
    "transforms.dropTopicPrefix.type":"org.apache.kafka.connect.transforms.RegexRouter",
    "transforms.dropTopicPrefix.regex":".*",
    "transforms.dropTopicPrefix.replacement":"my-topic"
  }'
  </code></pre>
  
  <p>For some reason, when I consume messages, I'm getting a weird key:</p>
  
  <pre><code> "_id": {
  "_data": "825DFD2A53000000012B022C0100296E5A1004060C0FB7484A4990A7363EF5F662CF8D465A5F6964005A1003F9974744D06AFB498EF8D78370B0CD440004"
    }
  </code></pre>
  
  <p>I have no Idea where did it come from, My mongo document's _id is UUID, When I'm consuming messages, I was expected to see the documentKey field at my consumer key.</p>
  
  <p>Here is a message example of what the connector published into kafka:</p>
  
  <pre><code>{
    "_id": {
      "_data": "825DFD2A53000000012B022C0100296E5A1004060C0FB7484A4990A7363EF5F662CF8D465A5F6964005A1003F9974744D06AFB498EF8D78370B0CD440004"
    },
    "operationType": "replace",
    "clusterTime": {
      "$timestamp": {
        "t": 1576872531,
        "i": 1
      }
    },
    "fullDocument": {
      "_id": {
        "$binary": "+ZdHRNBq+0mO+NeDcLDNRA==",
        "$type": "03"
      },
      ...
    },
    "ns": {
      "db": "security",
      "coll": "users"
    },
    "documentKey": {
      "_id": {
        "$binary": "+ZdHRNBq+0mO+NeDcLDNRA==",
        "$type": "03"
      }
    }
  }
  </code></pre>