Fix delete cookie test

Author: aguspe

rb/lib/selenium/webdriver/common/manager.rb

@@ -79,7 +79,7 @@ def cookie_named(name)
       #
 
       def delete_cookie(name)
-        raise ArgumentError, 'Cookie name cannot be null or empty' if name.nil? || name.strip.empty?
+        raise ArgumentError, 'Cookie name cannot be null or empty' if name.nil? || name.to_s.strip.empty?
 
         @bridge.delete_cookie name
       end