Use null as default values rather than false

Also made archive() in the ArchiveCommand protected as it does not need
to be used from the outside. The ArchiveManager can be used instead.

src/Composer/Command/ArchiveCommand.php

@@ -64,7 +64,7 @@ EOT
         );
     }
 
-    public function archive(IOInterface $io, $packageName = false, $version = false, $format = 'tar', $dest = '.')
+    protected function archive(IOInterface $io, $packageName = null, $version = null, $format = 'tar', $dest = '.')
     {
         $config = Factory::createConfig();
         $factory = new Factory;
@@ -91,7 +91,7 @@ EOT
         return 0;
     }
 
-    protected function selectPackage(IOInterface $io, $packageName, $version = false)
+    protected function selectPackage(IOInterface $io, $packageName, $version = null)
     {
         $io->write('<info>Searching for the specified package.</info>');