Keys in dict and keys not in dict

[INADA Naoki]

>     expected = {"foo", "bar", "spam"}
>     missing = expected - set(json.keys())
>

dict.keys() returns set-like object.
So `missing = expected - json.keys()` works fine, and it's more efficient.

-- 
INADA Naoki  <songofacandy at gmail.com>