Fixed too much memory usage for s3 streaming

Added memory usage tests as well for streaming uploads and downloads.  The actual snippet of code that I changed was in my original pull request before I rebased off of develop.  I added test for memory usage for both streaming uploads and streaming downloads so that this does not happen again.

Fixes https://github.com/aws/aws-cli/issues/923

cc @jamesls @danielgtaylor 

This should be written out to a file.  You do not want to create 200MB of data in the python process running the test.