<!-- 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)

...

pass
def make_http_request(url, data=None, headers=None, method='GET'):
    if is_string(method):
        method = requests.__dict__[method.lower()]
    return method(url, headers=headers, data=data, auth=NetrcBypassAuth(), verify=False)
class SafeStringIO(io.StringIO):
    """ Safe StringIO implementation that doesn't fail if str is passed in Python 2. """
    def write(self, obj):
        if six.PY2 and isinstance(obj, str):
            obj = obj.decode('unicode-escape')
        return super(SafeStringIO, self).write(obj)
def clean_cache(file_pattern=CACHE_FILE_PATTERN,
        last_clean_time=last_cache_clean_time, max_age=CACHE_MAX_AGE):
    mutex_clean.acquire()
    time_now = now()
    try:
        if last_clean_time['time'] > time_now - CACHE_CLEAN_TIMEOUT:
            return
        for cache_file in set(glob.glob(file_pattern)):
            mod_time = os.path.getmtime(cache_file)
            if time_now > mod_time + max_age:
                rm_rf(cache_file)
        last_clean_time['time'] = time_now
    finally:
        mutex_clean.release()
    return time_now
def truncate(data, max_length=100):