Set the correct content type for gzipped files with the .gz ext

[dannyfallon]

Using the gzip plugin it's possible to specify a keep option
which will maintain both the original uncompressed file and save
the gzipped version with a .gz suffix. Amazon S3 does not detect
content type or encoding so this is all managed in this plugin via
the mime package. The default mime type is application/octet-stream
which is exactly what is detected for a .gz files. Uploading
.js.gz and .css.gz files without their correct content types
blows up some browsers with syntax errors (e.g. Safari)

This PR makes the uploader a little smarter. If we detect a file is
gzipped and has a .gz extension we strip the extension and look for
the non-gzipped file in the file path list. If it's found, we'll do
a mime lookup for the non-gzipped file and use that content type
and encoding instead. If a non-gzipped counterpart isn't found we
will still upload the gzipped file as an application/octet-stream
so this shouldn't break any valid gzip assets that aren't compressed
as part of the gzip plugin in the pipeline.

[ghedamat]

thanks @dannyfallon for this!

LGTM at first pass but would love @achambers to confirm :)

[lukemelia]

Thanks @dannyfallon!