[#1665] Fix bug in helpers.change_config

The problem was that if the method being decorated raised an Exception, we
didn't reset the config to their original values. This patch guarantees that
we'll reset it.

ckan/new_tests/helpers.py

@@ -148,11 +148,13 @@ def decorator(func):
         def wrapper(*args, **kwargs):
             _original_config = config.copy()
             config[key] = value
+            return_value = None
 
-            return_value = func(*args, **kwargs)
-
-            config.clear()
-            config.update(_original_config)
+            try:
+                return_value = func(*args, **kwargs)
+            finally:
+                config.clear()
+                config.update(_original_config)
 
             return return_value
         return nose.tools.make_decorator(func)(wrapper)