[JENKINS-16215] Avoiding unnecessary downloads in FilePath.installIfNecessaryFrom

The updated version uses the locally cached timestamp, '.timestamp', to set the 'If-Modified-Since' header. 304 Not Modified responses are dealt with by aborting the installation.

The commit also contains two unit tests.
- If-Modified-Since is set, and installation is aborted because of '304 Not Modified'
- Installation still works as expected for other responses