Support for Content-Encoding Header

[matthewadowns]

Currently when specifying Content-Encoding: gzip, the header field gets renamed to x-amz-meta-content-encoding.

Chrome still knows how to render a gzipped file, but other browsers do not. The AWS CLI supports this feature, but I'm trying to avoid the overhead of that tool in my build process, but now I'm having to go manually edit the header fields after upload.