Fix required nested serializers

Fix required nested serializers

Added check on `None` in `to_representation` methods

Added check on None in to_representation methods in:
* CharField
* IntegerField
* FloatField
* DictField

Rename `fail` -> `fail_validate` in fields

0.3.12

rename field

Append fail method in fields

0.3.11

added method fail into fields. Rename old method fail to fail_field_v…

Fixed the argument source

Fixed the argument source in serializer fields into method to_representation.

Added the argument `allow_none` to serializer fields.

Added the argument allow_none to serializer fields. If set to True, skips None in the data field.

Fix aiohttp dispatch method

Fix aiohttp dispatch method. Current arguments for create response object

Change asyncio.corutine to async/await

Change asyncio.corutine to async/await

Fix remove fields from parent serializers

Fix for current remove fields from base serializer.

class Ser(Serializer):
    field_one = CharField()

class TwoSer(Ser):
    field_one = None

Fixes bugs for Aiohttp and serializers

• Fix for current work source attribute for base fields class.
• Fix exceptions __str__ method.
• Fix GenericViews MRO.
• Added unbound response_class from all views.
• Fix ValidationError arguments in views.