googleapiclient.http.DEFAULT_CHUNK_SIZE should be larger by default. #481

The default chunk size of 512kb is too small, and leads to slower-than-needed transfers, eg googlecolab/backend-container#1.

Users can of course set the value larger as they'd like, but I suspect increasing it would lead to a better experience for users who don't know that there's a knob for them to twiddle.