title: How to update an item in Dynamodb of type String Set (SS)?
body: <p>I have created a attribute of type String Set.
  When I create the Item and assign an attribute of type SS everything works.
  But when I try to update this attribute, the data type changes to a list ("L").</p>
  
  <p>I try this:</p>
  
  <pre><code>qw = new AWS.DynamoDB.DocumentClient();
  
  var params = {
    TableName : "myTable",
    Key: {
      "id": somekey
    },
    UpdateExpression: "set ssvar= :arrp",
    ExpressionAttributeValues: {
       ":arrp": [ "test", "test2" ]
    }
  };
  
  qw.update (etc.)
  </code></pre>
  
  <p>This leads to a change in datatype in dynamodb and in stead of a string set I get a list:</p>
  
  <pre><code>"ssvar": {
              "L": [
                  {
                      "S": "test"
                  }, 
                  {
                      "S": "test2"
                  }
              ]
          }
  </code></pre>
  
  <p>I have tried all kinds of solutions like below but always my datatype gets changed.</p>
  
  <pre><code>  ExpressionAttributeValues: {
       ":arrp": 
          "SS": [ "test", "test2" ]
    }
  </code></pre>
  
  <p>How can I update an attribute of type string set?</p>