Serialize default values in oneofs when calling to_dict() or to_json() (#110)

* Serialize default values in oneofs when calling to_dict() or to_json()

This change is consistent with the official protobuf implementation. If
a default value is set when using a oneof, and then a message is
translated from message -> JSON -> message, the default value is kept in
tact. Also, if no default value is set, they remain null.

* Some cleanup + testing for nested messages with oneofs

* Cleanup oneof_enum test cases, they should be fixed

This _should_ address:
https://github.com/danielgtaylor/python-betterproto/issues/63

* Include default value oneof fields when serializing to bytes

This will cause oneof fields with default values to explicitly be sent
to clients. Note that does not mean that all fields are serialized and
sent to clients, just those that _could_ be null and are not.

* Remove assignment when populating a sub-message within a proto

Also, move setattr out one indentation level

* Properly transform proto with empty string in oneof to bytes

Also, updated tests to ensure that which_one_of picks up the set field

* Formatting betterproto/__init__.py

* Adding test cases demonstrating equivalent behaviour with google impl

* Removing a temporary file I made locally

* Adding some clarifying comments

* Fixing tests for python38

tests/inputs/google_impl_behavior_equivalence/google_impl_behavior_equivalence.proto

@@ -0,0 +1,13 @@
+syntax = "proto3";
+
+message Foo{
+  int64 bar = 1;
+}
+
+message Test{
+  oneof group{
+    string string = 1;
+    int64 integer = 2;
+    Foo foo = 3;
+  }
+}

tests/inputs/google_impl_behavior_equivalence/test_google_impl_behavior_equivalence.py

@@ -0,0 +1,55 @@
+import pytest
+
+from google.protobuf import json_format
+import betterproto
+from tests.output_betterproto.google_impl_behavior_equivalence import (
+    Test,
+    Foo,
+)
+from tests.output_reference.google_impl_behavior_equivalence.google_impl_behavior_equivalence_pb2 import (
+    Test as ReferenceTest,
+    Foo as ReferenceFoo,
+)
+
+
+def test_oneof_serializes_similar_to_google_oneof():
+
+    tests = [
+        (Test(string="abc"), ReferenceTest(string="abc")),
+        (Test(integer=2), ReferenceTest(integer=2)),
+        (Test(foo=Foo(bar=1)), ReferenceTest(foo=ReferenceFoo(bar=1))),
+        # Default values should also behave the same within oneofs
+        (Test(string=""), ReferenceTest(string="")),
+        (Test(integer=0), ReferenceTest(integer=0)),
+        (Test(foo=Foo(bar=0)), ReferenceTest(foo=ReferenceFoo(bar=0))),
+    ]
+    for message, message_reference in tests:
+        # NOTE: As of July 2020, MessageToJson inserts newlines in the output string so,
+        # just compare dicts
+        assert message.to_dict() == json_format.MessageToDict(message_reference)
+
+
+def test_bytes_are_the_same_for_oneof():
+
+    message = Test(string="")
+    message_reference = ReferenceTest(string="")
+
+    message_bytes = bytes(message)
+    message_reference_bytes = message_reference.SerializeToString()
+
+    assert message_bytes == message_reference_bytes
+
+    message2 = Test().parse(message_reference_bytes)
+    message_reference2 = ReferenceTest()
+    message_reference2.ParseFromString(message_reference_bytes)
+
+    assert message == message2
+    assert message_reference == message_reference2
+
+    # None of these fields were explicitly set BUT they should not actually be null
+    # themselves
+    assert isinstance(message.foo, Foo)
+    assert isinstance(message2.foo, Foo)
+
+    assert isinstance(message_reference.foo, ReferenceFoo)
+    assert isinstance(message_reference2.foo, ReferenceFoo)