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# Type of request: This is a ...

[x] bug report
[ ] feature request

# Detailed description

In the `ReceiveMessageResult` of the SQS service, I saw this XML from localstack:

```
...
<MessageAttribute>
     <Name>encoding</Name>
    <Value>
        <DataType>String</DataType>
        <StringValue><![CDATA[gzip]]></StringValue>
    </Value>
</MessageAttribute>
...
```

The `<![CDATA[gzip]]>` is slightly different from AWS, which doesn't wrap the value in a `<!CDATA[[]]>` section. This shouldn't be a problem for XML parsers that accept both plain characters and CDATA sections for this element, but the XML parser I was using did not, which causes an error when running the code against localstack but not when running the code against AWS. There is obviously a bug in the XML parser as well, but I thought that I should at least post this here since it is different than what AWS SQS does.

## Expected behavior

MessageAttribute::Value elements should not be wrapped in `CDATA` sections.

## Actual behavior

MessageAttribute::Value elements all seem to be wrapped in `CDATA` sections.

# Steps to reproduce

Send a message to the SQS service which contains some custom message attributes, then make a ReceiveMessageRequest.

## Command used to start LocalStack

`docker run --env "SERVICES=sqs" localstack/localstack`

## Client code (AWS SDK code snippet, or sequence of "awslocal" commands)

...

queue_name = 'queue-%s' % short_uid()
        queue_url = self.client.create_queue(QueueName=queue_name)['QueueUrl']
        payload = {}
        attrs = {'attr1': {'StringValue': 'val1', 'DataType': 'String'}}
        self.client.send_message(QueueUrl=queue_url, MessageBody=json.dumps(payload),
                                 MessageAttributes=attrs)
        result = self.client.receive_message(QueueUrl=queue_url, MessageAttributeNames=['All'])
        messages = result['Messages']
        self.assertEquals(messages[0]['MessageAttributes'], attrs)
        
        self.client.delete_queue(QueueUrl=queue_url)
    def test_send_message_with_invalid_string_attributes(self):
        queue_name = 'queue-%s' % short_uid()
        queue_url = self.client.create_queue(QueueName=queue_name)['QueueUrl']
        payload = {}
        
        
        attrs = {'attr1': {
            'StringValue': 'invalid characters, %s, %s, %s' % (chr(8), chr(11), chr(12)), 'DataType': 'String'
        }}
        with self.assertRaises(Exception):
            self.client.send_message(QueueUrl=queue_url, MessageBody=json.dumps(payload),
                                     MessageAttributes=attrs)
        
        self.client.delete_queue(QueueUrl=queue_url)
    def test_dead_letter_queue_config(self):