conditional GET to npm group repository returns status 404 instead of 304 or 200

When a Nexus 3 instance proxy's a repository it sends conditional GET requests for npm metadata to the remote.  These fail if the remote is a group repository in another Nexus 3 instance.

Reproduce case:

1. Configure an npm proxy repository to https:registry.npmjs.org in Nexus 3.5.0
2. Create an npm group repository, add the proxy repo into it as a member
3. Request npm metadata through the group, such as http://localhost:8081/repository/npm-group/ansi-regex
4. Now request it again with a conditional GET - That will return a 404 response:

   $ curl -v -H "If-Modified-Since: Fri, 04 Aug 2017 16:45:35 GMT" http://localhost:8081/repository/npm-group/ansi-regex
   

Expected

Nexus should return 304 (not modified) IF
the combined/merged metadata for all group repository members is no different ( not updated ) as compared to the conditional get conditions as applied by general standards.

https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.25 OBSOLETE
https://tools.ietf.org/html/rfc7232 - CURRENT conditional gets
https://tools.ietf.org/html/rfc7230#section-3.2 - message headers general

Note: Sending the same conditional GET directly to the remote proxy returns 304.

Outcome

Group repo requests will return 200 status + the most recent package metadata content instead of 404 now.
Proper 304 response to conditional gets is a desired future improvement.