<!-- 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

Use localstack as AWS kinesis service mock and try to put a record with raw string data

```python
client.put_record(StreamName=stream_name, Data="raw_string", PartitionKey="MyParticonKey")
```

Then get the record using `subscribe_to_shard`

```python
result = self._client.subscribe_to_shard(
       ConsumerARN=self._consumer_arn, ShardId='shardId-000000000000', StartingPosition=start_position)
event = next(event_stream_iterator.__iter__())
```

And there is a `binascii.Error: Incorrect padding`. Backtrace:

```python
> /root/watercube/ms/aws/kds_consumer.py(188)_handle_event_stream()                                                                                                                                                                                           [0/1969]
-> event = next(event_stream_iterator.__iter__())
  /usr/local/lib/python3.7/site-packages/botocore/eventstream.py(571)__iter__()
-> parsed_event = self._parse_event(event)
  /usr/local/lib/python3.7/site-packages/botocore/eventstream.py(584)_parse_event()
-> parsed_response = self._parser.parse(response_dict, self._output_shape)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(242)parse()
-> parsed = self._do_parse(response, shape)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(644)_do_parse()
-> final_parsed[event_type] = self._do_parse(response, event_shape)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(648)_do_parse()
-> self._parse_payload(response, shape, shape.members, final_parsed)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(689)_parse_payload()
-> body_parsed = self._parse_shape(shape, original_parsed)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(302)_parse_shape()
-> return handler(shape, node)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(572)_handle_structure()
-> raw_value)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(302)_parse_shape()
-> return handler(shape, node)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(310)_handle_list()
-> parsed.append(self._parse_shape(member_shape, item))
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(302)_parse_shape()
-> return handler(shape, node)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(572)_handle_structure()
-> raw_value)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(302)_parse_shape()
-> return handler(shape, node)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(586)_handle_blob()
-> return self._blob_parser(value)
  /usr/local/lib/python3.7/site-packages/botocore/parsers.py(215)_default_blob_parser()
-> return base64.b64decode(value)
  /usr/local/lib/python3.7/base64.py(87)b64decode()
-> return binascii.a2b_base64(s)
```

While using AWS service endpoint, this does not happen.

## Expected behavior

`subscribe_to_shard` could properly decode the data returned from localstack when the record is put as string

## Actual behavior

`subcribe_to_shard` gets decode error

# Steps to reproduce

## Command used to start LocalStack

Start the localstack container using docker-compose,

```yaml
version: '2.4'
services:
    aws: 
         image: localstack/localstack:0.12.10
         restart: always
         ports: 
           - "4566"
         enviroments:
           - LOCALSTACK_SERVICES=s3,kinesis
           - LOCALSTACK_DEBUG=1
           - LOCALSTACK_HOSTNAME=aws
```

## Client code (AWS SDK code snippet, or sequence of "awslocal" commands)

```python
client.put_record(StreamName=stream_name, Data="raw_string", PartitionKey="MyParticonKey")
```

```python
result = self._client.subscribe_to_shard(
       ConsumerARN=self._consumer_arn, ShardId='shardId-000000000000', StartingPosition=start_position)
event = next(event_stream_iterator.__iter__())
```
- boto3==1.9.134
- botocore==1.12.253
import base64
import logging
import re
import unittest
from datetime import datetime
from time import sleep
import cbor2
import pytest
import requests
from localstack import config, constants
from localstack.utils.aws import aws_stack
from localstack.utils.common import poll_condition, retry, select_attributes, short_uid
from localstack.utils.kinesis import kinesis_connector
class TestKinesis(unittest.TestCase):
    def test_stream_consumers(self):
        client = aws_stack.create_external_boto_client("kinesis")
        stream_name = "test-%s" % short_uid()
        stream_arn = aws_stack.kinesis_stream_arn(stream_name)
        def assert_consumers(count):