Add --compressed to curl flags to match default Accept header

Splitting this out from #250.

The default singleton instance of `Alamofire.Manager` uses a configuration that adds compression to the `Accept-Encoding` header. curl's `--compressed` option does the same thing, but also will decompress the response. Indeed, it will end up having no effect on the request as the `-H` flag later on will override the `Accept-Encoding` header from the `--compressed` option.

Piping the response to `gunzip` or similar will be difficult, as the curl flags also include `-i`, putting headers in the response output.