schema fails to deserialize fields that are optional

[eliwe]

HI,

schema().loads failed to load Optional fields correctly. It raises:
marshmallow.exceptions.ValidationError: {'b': ['Field may not be null.']}

Example code:

from typing import Optional
from dataclasses import dataclass
from dataclasses_json import dataclass_json

@dataclass_json
@dataclass
class A:
    a: int
    b: Optional[int]

print(A.from_json('{"a": 4, "b": 5}'))
print(A.from_json('{"a": 4, "b": null}'))
print(A.schema().loads('{"a": 4, "b": 5}'))
print(A.schema().loads('{"a": 4, "b": null}'))

The last line raises ValidationError.
I am using version 0.2.1.

Thanks.

[lidatong]

Thanks for reporting this. Fixed in #66