Using `captureAWSClient` over directly referenced dynamodb client fails

The following code does not work:

```js
const DynamoDB = require('aws-sdk/clients/dynamodb')
const xray = require('aws-xray-sdk-core')
const ddb = xray.captureAWSClient(new DynamoDB())
```

In theory, it should be just the same as encapsulating `new AWS.DynamoDB()` coming straight from `aws-sdk`, but it does not work.

This is specially problematic in AWS Lambda environment, where loading more than you need makes you use more memory and more time.