Fix checking enum values.

[dpnova]

This fixes graphql-python/graphene#547 for me. I'm not sure how it impacts native enums though.

[dpnova]

There's a lot of broken tests here... I'll go digging, I'd appreciate anyone reviewing the change first though.

[lincolnq]

This issue is also related to graphql-python/graphene#558.

I think the proposed PR isn't a great solution, as I think both the enum's value and the enum instance itself should be accepted. IMO, this test case should pass --

import graphene as gql

class Currency(gql.Enum):
    USD = 10
    GBP = 20

class Query(gql.ObjectType):
    currency1 = Currency()
    currency2 = Currency()
    def resolve_currency1(self, info):
        """Return a Currency value."""
        return 10
    def resolve_currency2(self, info):
        """Return a Currency instance."""
        return Currency.USD

schema = gql.Schema(query=Query)
assert schema.execute('{currency1}').data['currency1'] == 'USD' # works
assert schema.execute('{currency2}').data['currency2'] == 'USD' # fails, it's None

[dpnova]

Yeah the PR just fixes my case. There's a bug here, we just need the best approach.

[dpnova]

Use #140