Jira should send valid last-modified headers for .css & .js

Issue Summary

Jira sometimes sends an invalid last-modified header for .css and .js files.  While this header is not required to be set (or set validly) for regular functioning of Jira, it is part of http-standards compliance, specifically for caching software to use.  Sending an invalid header is known to cause problems with specific proxy/caching software.

Environment

Bug observed in:

• Jira 8.13.7
• Jira 8.13.10
• Jira 8.15.1
• Jira 8.20.1
• Confluence

Bug fixed in Jira 8.20.8+

Steps to Reproduce

1. Run Jira 8.13+
2. Connect to Jira over IBM Webseal proxy

Expected Results

Pages will load as expected.

Actual Results

Jira pages load, but do not load/display completely.  Page styling is missing, for example.  (.css and .js are failing to load)

Get requests for .css and .js components will have the Response header Last-Modified set to:

Wed, 31 Dec 1969 23:59:59 GMT

Since this is one second before the beginning of Unix Time (Thu, 01 Jan 1970 00:00:00 GMT), Webseal will fail to calculate whether the cache should be refreshed and will return an error code instead of content.

Workaround

Disable cache-control in Webseal. For more information on Content Caching & the Last Modified Header in Webseal, see this IBM article: Content Caching in IBM Webseal