Set Content-Length for tile responses (fixes chunked encoding for gzipped tiles) - #283

This PR sets the Content-Length header for tile responses. Normally ServeContent would set the Content-Length header, but a commit in the net/http package 3 years ago changed that for responses with Content-Encoding set.

But for pmtiles, the content is already gzipped, and we are just passing it, so not setting Content-Length doesn't make sense for us, especially that it breaks things like Cloudflare CDN which relies on Content-Length header being present to cache anything in its reserve.

• Cloudflare CDN architecture: https://developers.cloudflare.com/reference-architecture/architectures/cdn/#introducing-the-cloudflare-cdn:~:text=The%20Content%2DLength%20header%20must%20be%20used%20in%20the%20response%20header.
• Go commit introducing the change: golang/go@fdc21f3

Diff:
https://github.com/golang/go/blob/master/src/net/http/fs.go#L399-L426