[JENKINS-14336]

Fixing a file handle leak
[JENKINS-14336]

Fixing another file handle leak.

JarURLConnection leaks a file descriptor (of the underlying jar file) when checking the timestamp.
The code path is as follows:

	at java.io.FileInputStream.<init>(FileInputStream.java:107)
	at java.io.FileInputStream.<init>(FileInputStream.java:66)
	at sun.net.www.protocol.file.FileURLConnection.connect(FileURLConnection.java:70)
	at sun.net.www.protocol.file.FileURLConnection.initializeHeaders(FileURLConnection.java:90)
	at sun.net.www.protocol.file.FileURLConnection.getHeaderField(FileURLConnection.java:126)
	at sun.net.www.protocol.jar.JarURLConnection.getHeaderField(JarURLConnection.java:203)
	at java.net.URLConnection.getHeaderFieldDate(URLConnection.java:603)
	at java.net.URLConnection.getLastModified(URLConnection.java:532)
	at org.kohsuke.stapler.Stapler.serveStaticResource(Stapler.java:249)
	at org.kohsuke.stapler.Stapler.serveStaticResource(Stapler.java:263)
	at org.kohsuke.stapler.ResponseImpl.serveFile(ResponseImpl.java:152)
	at org.kohsuke.stapler.framework.adjunct.AdjunctManager.doDynamic(AdjunctManager.java:139)
	at sun.reflect.GeneratedMethodAccessor537.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)

This is a well-recognized problem according to Google Search, so I'm working around it by
not using JarURLConnection.getLastModified().