fixed issue 1736, Unable to create/re-create cache file error for cache.disk, thanks arglanir

VERSION

@@ -1 +1 @@
-Version 2.7.4-stable+timestamp.2013.10.30.08.58.58
+Version 2.7.4-stable+timestamp.2013.10.31.09.39.21

gluon/cache.py

@@ -279,8 +279,9 @@ class CacheOnDisk(CacheAbstract):
                 logger.error('corrupted cache file %s, will try rebuild it'
                              % self.shelve_name)
                 storage = None
-            if not storage and os.path.exists(self.shelve_name):
+            if storage is None:
-                os.unlink(self.shelve_name)
+                if os.path.exists(self.shelve_name):
+                    os.unlink(self.shelve_name)
                 storage = shelve.open(self.shelve_name)
             if not CacheAbstract.cache_stats_name in storage.keys():
                 storage[CacheAbstract.cache_stats_name] = {