s3 sync --exact-timestamps flag ignored for uploads

There is error-prone situation described in [AWS S3 Sync Issues](https://github.com/aws/aws-cli/issues/599) when same-sized files are failed to be updated.

To help users to avoid issues, `--exact-timestamps` flag has been added to s3 sync command with [this PR](https://github.com/aws/aws-cli/pull/824).

However, the implementation is enabled for [download operation only](https://github.com/aws/aws-cli/pull/824/files#diff-10ce2c9e43db96130c4f81f25199d238L200), i.e. when files are being copied from s3 to local.

Hence is case of uploading files from local to s3 this flag does not work (ignored). I think this is rather counterintuitive and should be changed to make the flag respected for all the commands.