title: DynamoDB simple UpdateItem throwing "The provided key element does not match the schema" ValidationException
body: <p>I'm trying to update a string field in a DynamoDB table (thread1) which only has Hashkey(threadId). Document with threadId = "AA" definitely exists and also has field1 attribute.</p>
  
  <p>I'm getting <strong>"The provided key element does not match the schema"</strong> ValidationException when POST of UpdateItem from API Gateway is invoked using the following Body Mapping Template.</p>
  
  <pre><code>{
    "TableName": "thread1",
    "Key": {
      "HashKeyElement": {
        "S": "AA"
      }
    },
    "AttributeUpdates": {
      "field1": {
        "Value": {
          "S": "Worked!"
        }
      }
    }
  }
  </code></pre>
  
  <p>I have also tried the same using UpdateExpression, which also gives the same error.</p>