title: Cognito custom claims missing with Amplify but not with Appsync Console
body: <p>I have the following resolver, allowing me to retrieve information about the current user company (companyId is added as a custom field on the cognito user pool). The field on cognito is set to mutable.</p>
  
  <p><code>
  {
      "version" : "2017-02-28",
      "operation" : "GetItem",
      "key": {
          "id" : $util.dynamodb.toDynamoDBJson($context.identity.claims.get("custom:companyId"))
      }
  }
  </code></p>
  
  <p>-</p>
  
  <p>This works fine when using the AWS AppSync interface (after login in) as the logs show:</p>
  
  <p><code>
  {
      "errors": [],
      "mappingTemplateType": "Request Mapping",
      "path": "[getMyClientCompany]",
      "resolverArn": "arn:aws:appsync:eu-west-1:261378271140:apis/rue25cac6jc6vfbhvu32sjafqy/types/Query/fields/getMyClientCompany",
      "transformedTemplate": "{\n    \"version\" : \"2017-02-28\",\n    \"operation\" : \"GetItem\",\n    \"key\": {\n        \"id\" : {\"S\":\"0c1c81db-a771-4856-9a30-d11bf8e3cab1\"}\n    }\n}",
      "context": {
          "arguments": {},
          "source": null,
          "result": null,
          "error": null,
          "outErrors": []
      },
      "fieldInError": false
  }
  </code></p>
  
  <p>-</p>
  
  <p>But doesn't work when the code comes from Amplify-js:</p>
  
  <p><code>
  {
      "errors": [],
      "mappingTemplateType": "Request Mapping",
      "path": "[getMyClientCompany]",
      "resolverArn": "arn:aws:appsync:eu-west-1:261378271140:apis/rue25cac6jc6vfbhvu32sjafqy/types/Query/fields/getMyClientCompany",
      "transformedTemplate": "{\n    \"version\" : \"2017-02-28\",\n    \"operation\" : \"GetItem\",\n    \"key\": {\n        \"id\" : {\"NULL\":null}\n    }\n}",
      "context": {
          "arguments": {},
          "source": null,
          "result": null,
          "error": null,
          "outErrors": []
      },
      "fieldInError": false
  }
  </code></p>
  
  <p>The key that should be "custom:companyId" is "NULL" now
  I imagine the issue is either with Amplify (version 0.4.8) or with the cognito user resolver for some reason</p>
  
  <p>Any idea what could be going on?</p>