Fixes #654 - ServletContext.getResourceAsStream("/") should return null

+ Jetty 9.2 (and earlier) used a Resource.getInputStream() implementation
   that would trigger an IOException on-construction (vs on-first-access)
   due its use of java.io.FileInputStream(File) which had a isDirectory
   check in its constructor.
 + Jetty 9.3 and onward uses java.nio.file.Files.newInputStream() which
   has the bad on-first-access behavior.
 + Changing the behavior of PathResource.getInputStream() to behave the
   same way as the prior FileResource.getInputStream(), as well as adding
   the Resource.isDirectory() check back into ContextHandler.getResourceAsStream(String)
   to prevent creation of the InputStream in the first place.

jetty-server/src/main/java/org/eclipse/jetty/server/handler/ContextHandler.java

@@ -2123,6 +2123,9 @@ public class ContextHandler extends ScopedHandler implements Attributes, Gracefu
                 if (url == null)
                     return null;
                 Resource r = Resource.newResource(url);
+                // Cannot serve directories as an InputStream
+                if(r.isDirectory())
+                    return null;
                 return r.getInputStream();
             }
             catch (Exception e)

jetty-util/src/main/java/org/eclipse/jetty/util/resource/PathResource.java

@@ -377,6 +377,13 @@ public class PathResource extends Resource
     @Override
     public InputStream getInputStream() throws IOException
     {
+        /* Mimic behavior from old FileResource class and its
+         * usage of java.io.FileInputStream(File) which will trigger
+         * an IOException on construction if the path is a directory
+         */
+        if (Files.isDirectory(path))
+            throw new IOException(path + " is a directory");
+
         return Files.newInputStream(path,StandardOpenOption.READ);
     }