Last-Modified not returned in header for migrated RAW artifacts.

Nexus 3.x does not return a Last-Modified header for migrated artifacts (migrated from Nexus 2.x)

$ curl --head localhost:8081/repository/jenkins-tools/jdk/jdk1.8.0_171.tar.gz 
HTTP/1.1 200 OK 
Date: Thu, 21 Jun 2018 11:22:30 GMT 
Server: Nexus/3.12.0-01 (PRO) 
X-Content-Type-Options: nosniff 
Content-Security-Policy: sandbox allow-forms allow-modals allow-popups allow-presentation allow-scripts allow-top-navigation 
Content-Type: application/x-gzip 
Content-Length: 190882219

If you upload an artifact to that Nexus 3.x instance, then the Last-Modified is returned.

$ curl --head localhost:8081/repository/jenkins-tools-test/jdk/jdk1.8.0_171.tar.gz 
HTTP/1.1 200 OK 
Date: Thu, 21 Jun 2018 11:22:24 GMT 
Server: Nexus/3.12.0-01 (PRO) 
X-Content-Type-Options: nosniff 
Content-Security-Policy: sandbox allow-forms allow-modals allow-popups allow-presentation allow-scripts allow-top-navigation 
Last-Modified: Thu, 21 Jun 2018 11:17:29 GMT 
Content-Type: application/x-gzip 
Content-Length: 190882219

Reproduced this with Raw repo. Works fine with Maven2