Default values are not serialized in oneofs when calling .to_dict() or .to_json() (which is different behaviour from official protobuf impl)

# Consider this protobuf message:

```
message OneOfDemo{
oneof value_type {
bool bool_value = 1;
int64 int64_value = 2;
}
}
```

# Using the official protobuf codegen and library (libprotoc 3.12.3):

```
from google.protobuf.json_format import MessageToDict

# Official protobuf:
print("True: ", json.dumps(MessageToDict(OneOfDemo(bool_value=True))))
print("False:", json.dumps(MessageToDict(OneOfDemo(bool_value=False))))
print("1: ", json.dumps(MessageToDict(OneOfDemo(int64_value=1))))
print("0: ", json.dumps(MessageToDict(OneOfDemo(int64_value=0))))

Output:
True: {"boolValue": true}
False: {"boolValue": false}
1: {"int64Value": "1"}
0: {"int64Value": "0"}
```

# Using betterproto (1.2.5)

```
# Generated code:
@dataclass
class OneOfDemoBetter(betterproto.Message):
bool_value: bool = betterproto.bool_field(1, group="value_type")
int64_value: int = betterproto.int64_field(2, group="value_type")

print("True: ", OneOfDemoBetter(bool_value=True).to_json())
print("False:", OneOfDemoBetter(bool_value=False).to_json())
print("1: ", OneOfDemoBetter(int64_value=1).to_json())
print("0: ", OneOfDemoBetter(int64_value=0).to_json())

Output:
True: {"boolValue": true}
False: {}
1: {"int64Value": "1"}
0: {}
```

This obviously leads to the following inconsistency:

```
od = OneOfDemoBetter(bool_value=False)
print(betterproto.which_one_of(od, "value_type"))

od2 = OneOfDemoBetter().from_json(od.to_json())
print(betterproto.which_one_of(od2, "value_type"))

Output:
('bool_value', False)
('', None)
```

# Misc

* Python 3.8.1
* I love what you are trying to do with this project btw - the official generated protobuf code is AWEFUL and this is a HUGE step forwards. This one difference in behaviour is blocking wholesale adoption of your project unfortunately
* Tangentially related ideas and observations
* Would it make sense to have values in the oneof be declared `Optional`? (ie: `bool_value: Optional[bool]`) since only one is expected to be set? Personally I would prefer this but I can understand if its getting too far away from the idea of "there are no nulls in proto3".
* `.to_json()` is missing the `casing` arg (and `include_default_values`) from `.to_dict()`
* Stretch/wishlist: personally I would find it super valuable if there were some way to default to `Casing.SNAKE` for all `.to_json()` operations automatically. Perhaps something like a default option you could specify on a per message basis?

Great work on this project! Keep it up!