[Core] Bug in stream download retry - implicit int to str

[annatisch]

@xiangyan99

This line raises an error on retry:
https://github.com/Azure/azure-sdk-for-python/blob/master/sdk/core/azure-core/azure/core/pipeline/transport/requests_basic.py#L123

If self.downloaded is supposed to be an int, then this line needs str(self.downloaded).
We also need to address this line:
https://github.com/Azure/azure-sdk-for-python/blob/master/sdk/core/azure-core/azure/core/pipeline/transport/requests_basic.py#L130
Which should probably be self.downloaded += len(chunk)

Otherwise self.downloaded is meant to be bytes/str, in which case it should probably not be initialized to 0?