Hello guys. I use S3 preSigned URL to upload file, but it return an error "The Content-MD5 you specified was invalid".
It happens only when I generate Presigned Url with contentMd5.However it works fine when I annotation the line '.withContentMd5(md5)'.
My file md5 is 32c295626872e465b3d0b590eb8e88a7.

```java
        AmazonS3 s3client = getDefaultS3Client();
        Date expiration = new Date(System.currentTimeMillis() + expiratMillis);
        GeneratePresignedUrlRequest generatePresignedUrlRequest =
                new GeneratePresignedUrlRequest(bucketName, s3FullPath)
                        .withMethod(HttpMethod.PUT)
                        .withExpiration(expiration)
                        .withKey(s3Key)
                        .withContentMd5(md5);
        String presignedUrl =  s3client.generatePresignedUrl(generatePresignedUrlRequest);
 ```


![postman](https://user-images.githubusercontent.com/4321439/60160341-bdfafd80-9827-11e9-8a26-03ec4c4d8faf.png)

Does S3 support checksum when using presigned url?
Could anybody give me a piece of advice to help me solve this? Thank you!
records = []
        i = 0
        for table_name in sorted(data['RequestItems'].keys()):
            for request in data['RequestItems'][table_name]:
                put_request = request.get('PutRequest')
                if put_request:
                    existing_item = self._thread_local('existing_items')[i]
                    keys = dynamodb_extract_keys(item=put_request['Item'], table_name=table_name)
                    if isinstance(keys, Response):
                        return keys
                    new_record = clone(record)
                    new_record['eventName'] = 'INSERT' if not existing_item else 'MODIFY'
                    new_record['dynamodb']['Keys'] = keys
                    new_record['dynamodb']['NewImage'] = put_request['Item']
                    if existing_item:
                        new_record['dynamodb']['OldImage'] = existing_item
                    new_record['eventSourceARN'] = aws_stack.dynamodb_table_arn(table_name)
                    records.append(new_record)
                delete_request = request.get('DeleteRequest')
                if delete_request:
                    keys = delete_request['Key']
                    if isinstance(keys, Response):
                        return keys
                    new_record = clone(record)
                    new_record['eventName'] = 'REMOVE'