Make enum types pass validation

[svenschwermer]

marshmallow 3.0.0 doesn't perform validation during dump[s]. In order to
get the equivalent behaviour as before, validate must be called. However,
this fails with "Enum name must be string" for the following MWE:

class MyEnum(Enum):
    a = 1

class MySchema(Schema):
    my_field = EnumField(MyEnum)

errors = MySchema().validate({"my_field": MyEnum.a})
assert not errors

Internally, validate calls _do_load which invokes _deserialize aon all
fields. If there is nothing to deserialize, i.e. the type is already
correct, don't do anything.

Signed-off-by: Sven Schwermer sven.schwermer@disruptive-technologies.com