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
import os
import localstack_client.config
VERSION = '0.10.4'
REGION_LOCAL = 'local'
ENV_DEV = 'dev'
DEFAULT_PORT_APIGATEWAY_BACKEND = 4566
DEFAULT_PORT_KINESIS_BACKEND = 4565
DEFAULT_PORT_DYNAMODB_BACKEND = 4564
DEFAULT_PORT_S3_BACKEND = 4563
DEFAULT_PORT_SNS_BACKEND = 4562
DEFAULT_PORT_SQS_BACKEND = 4561
DEFAULT_PORT_ELASTICSEARCH_BACKEND = 4560
DEFAULT_PORT_CLOUDFORMATION_BACKEND = 4559
DEFAULT_PORT_STEPFUNCTIONS_BACKEND = 4558
DEFAULT_PORT_IAM_BACKEND = 4557
DEFAULT_PORT_EC2_BACKEND = 4556
DEFAULT_PORT_WEB_UI = 8080
LOCALHOST = 'localhost'
LOCALSTACK_MAVEN_VERSION = '0.1.22'
DEFAULT_SERVICE_PORTS = localstack_client.config.get_service_ports()
BIND_HOST = '0.0.0.0'
if 'TEST_AWS_ACCOUNT_ID' not in os.environ:
    os.environ['TEST_AWS_ACCOUNT_ID'] = '000000000000'