New Rule Proposal: warning for missing file modification date

[owades]

Summary:

New rule: Check whether each RT feed header includes file modification date, as per the following best practice:

The web-server hosting GTFS Realtime data should be configured to correctly report the file modification date (see HTTP/1.1 
- Request for Comments 2616, under Section 14.29) so consumers can leverage the If-Modified-Since HTTP header. This 
saves producers and consumers bandwidth by avoiding transferring feed contents that haven't changed.

Severity: Warning

Expected behavior:

If the file modification date header is not present for any RT feed, throw a warning.

[briandonahue]

Investigating this. Currently validation does not begin until after the file is downloaded locally, therefore the request header information is not available. Will discuss possible implementations with @KClough and update the issue once we've determined an approach.

[briandonahue]

@bdferris-v2 It seems I can check for this in BackgroundTask.java where the request is made, but I'm looking for feedback on if/how to store that as metadata to validate and generate a notice during the validation phase?