Expose acceptsGzip

[wyattjoh]

I'd like to use this package with my custom web application wrapper, but I can't match the functionality provided by the NewGzipLevelHandler without the acceptsGzip function exposed: https://github.com/NYTimes/gziphandler/blob/fb3533722e14198abe471546c9798fd556531451/gzip.go#L174

[adammck]

I wouldn't be against making acceptsGzip/parseEncodings/parseCoding public, but we should move them to a subpackage if we do so. For the latter two, we might want to consider dropping our impl in favor of gddo/httputil/header.ParseAccept anyway, which would make the former trivial.

[glasser]

In general it's hard to use the exported GzipResponseWriter directly because of this issue but also because there's no way to set its options from outside the package.