Enum default_value serialization issue

Originally reported in graphene issue #691

Argument default_value serializes enum value instead of name. graphql spec does not support enum values and graphql-js serializes enums to the name not value. graphql-python is not consistent with graphql-js. I attempted to find a workaround by setting the enum value to a string of the name but teh result is a string that includes escaped quotes.

Using the name of the enum doesn't work:

class MyEnum(graphene.Enum):
    FOO=0
    BAR=1

foobar = graphene.List(FooBar, my_enum=graphene.Argument(MyEnum, default_value=MyEnum.FOO.name))

This results in the defaultValue being "\"FOO\"". This does not follow the graphql spec for enums and breaks with third party introspection. Specifically i'm trying to stitch a graphene remote schema and the invalid defaultValue results in a null returned in the merged schema (using graphql-tools).

Using a string value doesn't work either:

class MyEnum(graphene.Enum):
    FOO="FOO"
    BAR="BAR"

This also doesn't work with using MyEnum.FOO.value for default_value nor setting it specifically to the static string "FOO".

I tried looking for how default_value gets resolved for enums but haven't been able to track it down. I think if we can just get the string value for default_value to not include the escaped string quotes then my schema stitching will work. I haven't been able to figure out how to define an enum so that this will happen.

首先复现 enum 参数 default_value 的示例，并跟踪默认值在 schema introspection 期间是如何序列化的。将结果与 graphql-js 的行为进行比较；当 enum 默认值以其名称输出、没有转义的字符串引号，并且可以被 graphql-tools 用于 schema stitching 时，即表示完成。