Automatically ungzip Transfer-Encoding: gzip files.

If we upload a file to S3 using gzip compression and zlib, then try to use `getFile` with it, we have to then pipe it through gunzip per #226.

Do you think it'd be appropriate to add a new method such as `client.getGunzippedFile` which would return the gunzipped file?

Start by reading the existing getFile behavior and the discussion in #226, then determine where gzip-encoded S3 responses are handled. The change is done when a clearly defined getGunzippedFile-style entry point returns the decompressed file and its behavior is covered by the relevant tests.

Written by the indexing model from the issue text.