Bug: Compound Key Definition doesn't work with EnumField

When using Enum Field as a key in containing a compound statement, validation fails and doesn't work (raises AssertionError: Invalid compound key definition for type). But the same works if there is no compound key, only enum key.

Following sample can be used to reproduce the issue:
```python
class OrgTypeEnum(Enum):
LOCAL = 0
GLOBAL = 1

class Organization(ObjectType):
registration_number = ID()
organization_type = OrgTypeEnum()

@key("id organization { registration_number organization_type } ")
class User(ObjectType):
id = ID()
organization = Field(Organization)
```