Fixed incorrect error filtering when getting Last-Modified from jar resources.

JarEntry#getTime returns -1 if the modification time is not specified (not 0).
Refactored the handling of Last-Modified.

It doesn't need to specify the path to the entry.
It is more simple to get the entry from JarURLConnection directly.
Fixed missing last-modified header in Test and Dev.
Merge pull request #1256 from tototoshi/fix-assets-last-modified

 Fixed incorrect error filtering when getting Last-Modified from jar resources.