Url Mime Caching

[kernelkind]

If the mime type hosted at a URL can't be inferred from the URL, a HEAD request is made and the result is cached. Every so often the data structure is saved to disk

[jb55]

Review for "Url Mime Caching" #743
by kernelkind

Here's a brief review of your PR:

Handling the HEAD Response: There's a subtlety in ehttp_get_mime_type. When the server returns an error or fails to include a Content-Type header, the promise is
never completed (sender.send(...) is never invoked in the error branch). Because of that:

• The in_flight entry for that URL remains in the HashMap forever.
• "UrlMimes::get" will keep returning None for that URL indefinitely, since the promise never resolves or fails.
• Future attempts to retrieve a MIME type for that URL won't spawn another HEAD request, effectively leaving it stuck.

You likely want to call sender.send(...) in both success and failure paths so that the promise always completes, or otherwise remove the promise from the in_flight
Map.

[jb55]

shouldn't this be a part of imagecache? do we really want to prop drill this independently?