File.mtime doesn't return a useful value for a file in a jar #3812

Symptom

Sinatra AssetPack fails to get a valid Last-Modified time from an asset packaged in a JAR causing upstream processing by Rack::Deflater to fail.

Environment

jruby-9.0.3.0
Linux & Mac (when running from a jar archive)
Using Sinatra, Sinatra AssetPack, and Rack::Deflater

Expected Behavior

When Rack goes to read the static asset from the jar file via the classpath: scheme it should provide a File object that returns useful Time object for File.mtime, so that when the Last-Modified header is read and parsed upstream processing w/ Rack::Deflater it can appropriately parse the Time string.

Actual Behavior

It throws an ArgumentError @: https://github.com/headius/jruby/blob/master/lib/ruby/2.0/time.rb#L505

Another useful line just above the previous link in the stack : https://github.com/rack/rack/blob/master/lib/rack/deflater.rb#L58

What should File.mtime return for a file in a jar? The modified time for the file or at a minimum the modified time for the jar? Here's an example.

$ java -jar jruby-complete-9.0.3.0.jar -e 'load "META-INF/jruby.home/bin/jirb"'
>> File.exists?("classpath:META-INF/jruby.home/bin/jirb")
=> true
>> File.mtime("classpath:META-INF/jruby.home/bin/jirb")
=> 1969-12-31 16:00:00 -0800
>> File.mtime("classpath:META-INF/jruby.home/bin")
=> 1969-12-31 16:00:00 -0800