Ruby: fixed contents of SCRIPT_NAME.

Having the basename of the script pathname was incorrect.  While
we don't have something more accurate, the best thing to do is to
have it empty (which should be the right thing most of the time).

This closes #715 issue on GitHub.

The bug was introduced in git commit
0032543fa6
'Ruby: added the Rack environment parameter "SCRIPT_NAME".'.

test/test_ruby_application.py

@@ -44,7 +44,7 @@ class TestRubyApplication(TestApplicationRuby):
             'Request-Method': 'POST',
             'Request-Uri': '/',
             'Http-Host': 'localhost',
-            'Script-Name': 'config.ru',
+            'Script-Name': '',
             'Server-Protocol': 'HTTP/1.1',
             'Custom-Header': 'blah',
             'Rack-Version': '13',