make sure serializer is set

If you pass through a custom serializer, then `s` will be unset and will fail when being passed into _store. Example:

  File "/Users/andy/.virtualenvs/curling/lib/python2.7/site-packages/slumber/__init__.py", line 187, in __init__
    "serializer": s,
UnboundLocalError: local variable 's' referenced before assignment