Check for changes to operator attributes when getting the schema

[hdoupe]

This fixes a bug that occurs when you update an operator attribute like array_first after creating the Parameters instance and then call dump. The "schema" object returned will not have the updated operators.

Before:

In [1]: from paramtools import Parameters

In [2]: class Params(Parameters):
   ...:     defaults = {"schema": {"operators": {"array_first": True}}}
   ...: 

In [3]: params = Params()

In [4]: params.array_first = False

In [5]: params.dump()
Out[5]: 
{'schema': {'labels': {},
  'operators': {'label_to_extend': None,
   'uses_extend_func': False,
   'array_first': True},                          # <=== this should be False!
  'additional_members': {}}}

After fix:

In [1]: from paramtools import Parameters

In [2]: class Params(Parameters):
   ...:     defaults = {"schema": {"operators": {"array_first": True}}}
   ...: 

In [3]: params = Params()

In [4]: params.array_first = False

In [5]: params.dump()
Out[5]: 
{'schema': {'additional_members': {},
  'operators': {'uses_extend_func': False,
   'array_first': False,                          # <=== this is False now.
   'label_to_extend': None},
  'labels': {}}}