It looks like a duplicate of old issue https://github.com/localstack/localstack/issues/78
But now it's reproducible for me with the latest version of LocalStack. 
import re
import uuid
import xmltodict
from six.moves.urllib import parse as urlparse
from six.moves.urllib.parse import urlencode
from requests.models import Request, Response
from localstack import config
from localstack.config import HOSTNAME_EXTERNAL, SQS_PORT_EXTERNAL
from localstack.utils.common import to_str, md5
from localstack.utils.analytics import event_publisher
from localstack.services.awslambda import lambda_api
from localstack.utils.aws.aws_stack import extract_region_from_auth_header
from localstack.services.generic_proxy import ProxyListener
XMLNS_SQS = 'http://queue.amazonaws.com/doc/2012-11-05/'
SUCCESSFUL_SEND_MESSAGE_XML_TEMPLATE = """
    <?xml version="1.0"?>
    <SendMessageResponse xmlns="%s">
        <SendMessageResult>
            <MD5OfMessageAttributes>{message_attr_hash}</MD5OfMessageAttributes>
            <MD5OfMessageBody>{message_body_hash}</MD5OfMessageBody>
            <MessageId>{message_id}</MessageId>
        </SendMessageResult>
        <ResponseMetadata>
            <RequestId>00000000-0000-0000-0000-000000000000</RequestId>
        </ResponseMetadata>