Structured Config created from dataclass instance with default_factory does not retain user-assigned MISSING field.

[Jasha10]

Closely related to #830.

Describe the bug
Given a dataclass instance obj, the DictConfig object OmegaConf.structured(obj) created from obj should have fields that correspond to the fields of the original object obj.

Currently the above fails if obj has a field with a default_factory and with a user-assigned MISSING value, e.g. if the user creates obj like this:

obj = MyConf(fieldname=MISSING)

where MyConf.fieldname has a non-missing default_factory.

To Reproduce

from dataclasses import dataclass, field
from typing import List

from omegaconf import MISSING, OmegaConf


@dataclass
class ConfigWithLeaf:
    leaf: int = 1


input1 = ConfigWithLeaf(leaf=MISSING)
assert input1.leaf == MISSING                  # ok
conf1 = OmegaConf.structured(input1)
assert OmegaConf.is_missing(conf1, "leaf")     # ok


@dataclass
class ConfigWithList:
    lst: List[int] = field(default_factory=lambda: [1, 2, 3])


input2 = ConfigWithList(lst=MISSING)
assert input2.lst == MISSING                   # ok
conf2 = OmegaConf.structured(input2)
assert OmegaConf.is_missing(conf2, "lst")      # AssertionError

Expected behavior
All four assertions above should pass.

Additional context

• OmegaConf version: master branch