Code used to add Last-Modified from mbtiles mtime seems to be removed

[maloff1]

This commit f446948 from this pull request from 2016 #25 added Last-Modified header with date from mbtiles.

This code seems to be missing from the current codebase.

I think this header is useful for caching, and before I start hacking and make a pull request to return alike code, can someone please point out if removal happened because of well thought out considerations, or this was an omission during some later rewrite?

[brendan-ward]

Looks like this got dropped quite some time ago; I don't have a clear memory as to why. It might have been part of migrating from having the handlers at the top level and handled by echo to moving them to the handlers package and handling via Go route handlers.

The timestamp of a tileset is still available as part of the MBtiles struct