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 

Would you mind updating the docstring with this new value.  Especially how it's different from `input_data`.