If I don't use multibyte strings, the same MD5OfMessageAttibutes are returned from SQS on localstack and AWS.

```
aws --endpoint-url http://localhost:4576 sqs send-message --queue-url http://localhost:4576/queue/dummy --message-body 'foo' --message-attributes '{ "XXX": {"DataType": "String", "StringValue": "xxx"}}'
{
    "MD5OfMessageBody": "acbd18db4cc2f85cedef654fccc4a4d8",
    "MD5OfMessageAttributes": "37819bc3d58fcdbf7bc9aa49ff38a4e6",
    "MessageId": "867a01da-61f4-4841-862f-10e04b4741cf"
}

aws sqs send-message --queue-url https://sqs.ap-northeast-1.amazonaws.com/this_is/dummy --message-body 'foo' --message-attributes '{ "XXX": {"DataType": "String", "StringValue": "xxx"}}'
{
    "MD5OfMessageBody": "acbd18db4cc2f85cedef654fccc4a4d8",
    "MD5OfMessageAttributes": "37819bc3d58fcdbf7bc9aa49ff38a4e6",
    "MessageId": "49b05443-d174-4766-93b4-dd6c08daa29c"
}
```

But, using multibyte strings, such as emoji, Japanese , causes different MD5OfMessageAttributes.

```
# If emoji 😇 used

aws --endpoint-url http://localhost:4576 sqs send-message --queue-url http://localhost:4576/queue/dummy --message-body 'foo' --message-attributes '{ "XXX": {"DataType": "String", "StringValue": "😇"}}'
{
    "MD5OfMessageBody": "acbd18db4cc2f85cedef654fccc4a4d8",
    "MD5OfMessageAttributes": "068b4cc7d27c322bde50451db1f3b838",
    "MessageId": "3b6ec9b7-96df-44f8-987e-acf001d551e4"
}

aws sqs send-message --queue-url https://sqs.ap-northeast-1.amazonaws.com/this_is/dummy --message-body 'foo' --message-attributes '{ "XXX": {"DataType": "String", "StringValue": "😇"}}'
{
    "MD5OfMessageBody": "acbd18db4cc2f85cedef654fccc4a4d8",
    "MD5OfMessageAttributes": "23bf3e5b587065b0cfbe95761641595a",
    "MessageId": "0868914f-ac9c-41c6-a278-c7330acfa7dc"
}

# If Japanese あああ used

aws --endpoint-url http://localhost:4576 sqs send-message --queue-url http://localhost:4576/queue/dummy --message-body 'foo' --message-attributes '{ "XXX": {"DataType": "String", "StringValue": "あああ"}}'
{
    "MD5OfMessageBody": "acbd18db4cc2f85cedef654fccc4a4d8",
    "MD5OfMessageAttributes": "be67b7518e7e38cd41a336656a177fc7",
    "MessageId": "0b40c867-55d2-4b9c-a37f-89da8c10e2a4"
}

aws sqs send-message --queue-url https://sqs.ap-northeast-1.amazonaws.com/this_is/dummy --message-body 'foo' --message-attributes '{ "XXX": {"DataType": "String", "StringValue": "あああ"}}'
{
    "MD5OfMessageBody": "acbd18db4cc2f85cedef654fccc4a4d8",
    "MD5OfMessageAttributes": "244afccd2af83b4f0829298cda77cf3d",
    "MessageId": "f00d98e9-392f-43e3-bf0a-e3e78c758421"
}
```

Versions
localstack commit hash: 866b871f151298d45061cdf525669159725f43e6
aws cli: aws-cli/1.16.230 Python/3.7.4 Darwin/18.7.0 botocore/1.12.220
TEST_AWS_ACCOUNT_ID = os.environ['TEST_AWS_ACCOUNT_ID']
LOCALSTACK_ROOT_FOLDER = os.path.realpath(os.path.join(os.path.dirname(os.path.realpath(__file__)), '..'))
LOCALSTACK_VENV_FOLDER = os.path.join(LOCALSTACK_ROOT_FOLDER, '.venv')
if not os.path.isdir(LOCALSTACK_VENV_FOLDER):
    
    LOCALSTACK_VENV_FOLDER = os.path.realpath(os.path.join(LOCALSTACK_ROOT_FOLDER, '..', '..', '..'))
PATH_USER_REQUEST = '_user_request_'
DOCKER_IMAGE_NAME = 'localstack/localstack'
CONFIG_UPDATE_PATH = '/?_config_'
ENV_INTERNAL_TEST_RUN = 'LOCALSTACK_INTERNAL_TEST_RUN'
APPLICATION_AMZ_JSON_1_0 = 'application/x-amz-json-1.0'
APPLICATION_AMZ_JSON_1_1 = 'application/x-amz-json-1.1'
APPLICATION_JSON = 'application/json'
APPLICATION_X_WWW_FORM_URLENCODED = 'application/x-www-form-urlencoded'
TRUE_STRINGS = ('1', 'true', 'True')
FALSE_STRINGS = ('0', 'false', 'False')
LAMBDA_TEST_ROLE = 'arn:aws:iam::%s:role/lambda-test-role' % TEST_AWS_ACCOUNT_ID
ELASTICSEARCH_JAR_URL = 'https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-6.7.0.zip'
ELASTICSEARCH_PLUGIN_LIST = ['analysis-icu', 'ingest-attachment', 'analysis-kuromoji',
 'mapper-murmur3', 'mapper-size', 'analysis-phonetic', 'analysis-smartcn', 'analysis-stempel', 'analysis-ukrainian']
ELASTICSEARCH_DELETE_MODULES = ['ingest-geoip']
DYNAMODB_JAR_URL = 'https://s3-us-west-2.amazonaws.com/dynamodb-local/dynamodb_local_latest.zip'
ELASTICMQ_JAR_URL = 'https://s3-eu-west-1.amazonaws.com/softwaremill-public/elasticmq-server-0.14.2.jar'
STS_JAR_URL = 'https://repo1.maven.org/maven2/com/amazonaws/aws-java-sdk-sts/1.11.14/aws-java-sdk-sts-1.11.14.jar'
STEPFUNCTIONS_ZIP_URL = 'https://s3.amazonaws.com/stepfunctionslocal/StepFunctionsLocal.zip'
API_ENDPOINT = os.environ.get('API_ENDPOINT') or 'https://api.localstack.cloud/v1'
LOCALSTACK_WEB_PROCESS = 'LOCALSTACK_WEB_PROCESS'
LOCALSTACK_INFRA_PROCESS = 'LOCALSTACK_INFRA_PROCESS'
MOTO_ACCOUNT_ID = '123456789012'