<!-- Love localstack? Please consider supporting our collective:
👉  https://opencollective.com/localstack/donate -->

# 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)

...

if not port:
        port = config.EDGE_PORT
    if config.EDGE_PORT_HTTP:
        do_start_edge(config.EDGE_PORT_HTTP, use_ssl=False, asynchronous=True)
    if port > 1024 or is_root():
        return do_start_edge(port, use_ssl, asynchronous=asynchronous)
    
    class Terminator(object):
        def stop(self, quiet=True):
            try:
                url = 'http%s://localhost:%s' % ('s' if use_ssl else '', port)
                requests.verify_ssl = False
                requests.post(url, headers={HEADER_KILL_SIGNAL: 'kill'})
            except Exception:
                pass
    
    ensure_can_use_sudo()
    
    TMP_THREADS.append(Terminator())
    
    sudo_cmd = 'sudo '
    python_cmd = sys.executable
    cmd = '%sPYTHONPATH=.:%s %s %s %s' % (sudo_cmd, LOCALSTACK_ROOT_FOLDER, python_cmd, __file__, port)
    process = run(cmd, asynchronous=asynchronous)
    return process
if __name__ == '__main__':
    logging.basicConfig()
    start_edge(int(sys.argv[1]))