avoid double read from steam from curl when stream is small

[sbiscigl]

Description of changes:

Fixes a corner case in aws chunked streaming that lead to performance problems. we recently fixed a bug in aws chunked streaming that fixed it in curl. There was however a performance oversight. on a chunk less that the default chunk size, it would require two stream reads from curl instead of one. requiring two stream reads increased latency very slightly for smaller objects.

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[sbera87]

lgtm