Don't use None as the default value for optimizer as this also mean no optimizer!

doc/sandbox/debugging_with_stepmode.txt

@@ -17,10 +17,10 @@ purpose of it is to hack it to investigate what your own particular program is d
             predefined_optimizers)
 
     class StepMode(Mode):
-        def __init__(self, linker=None, optimizer=None):
+        def __init__(self, linker=None, optimizer='default'):
             if linker is None:
                 linker = config.linker
-            if optimizer is None:
+            if optimizer is 'default':
                 optimizer = config.optimizer
             def blah(i, node, th):
                 # This function will be run for each node in your compiled program.

theano/compile/mode.py

@@ -286,10 +286,10 @@ class Mode(object):
     predefined_modes.
     """
 
-    def __init__(self, linker=None, optimizer=None):
+    def __init__(self, linker=None, optimizer='default'):
         if linker is None:
             linker = config.linker
-        if optimizer is None:
+        if optimizer is 'default':
             optimizer = config.optimizer
         self.__setstate__((linker, optimizer))
 

theano/compile/profilemode.py

@@ -82,10 +82,10 @@ class Profile_Maker(FunctionMaker):
         return ret
 
 class ProfileMode(Mode):
-    def __init__(self, linker=None, optimizer=None):
+    def __init__(self, linker=None, optimizer='default'):
         if linker is None:
             linker = config.linker
-        if optimizer is None:
+        if optimizer is 'default':
             optimizer = config.optimizer
         message = ""
         profile_stats = {}