Memory leak - issue while downloading large files

Issue :
next_chunk method stores downloaded content to 'fd' file object. This downloaded content is stored in ram. As the file size increases, it consumes more RAM to store downloaded data. 

Reference - https://github.com/googleapis/google-api-python-client/blob/5399bd3bf34ef31302a77ff484ef0067a18709a6/googleapiclient/http.py#L755

Related issues:
#512
#198