equal: code doesn't distinguish between oneof fields when zero-valued

[misberner]

Because the comparison logic for oneof fields relies on the getters for the individual fields, it cannot differentiate between a field not being set, and a field being set to the zero value. While the nil checks allow distinguishing protos where one has a oneof field set to a zero value, while the other doesn't have any field in the oneof set, the code fails to distinguish protos where different fields in a oneof are set to the respective zero value.

Test case:

func TestEqualVT_Oneof_AbsenceVsZeroValue(t *testing.T) {
	a := &TestAllTypesProto3{
		OneofField: &TestAllTypesProto3_OneofUint32{
			OneofUint32: 0,
		},
	}
	b := &TestAllTypesProto3{
		OneofField: &TestAllTypesProto3_OneofString{
			OneofString: "",
		},
	}

	aJson, err := protojson.Marshal(a)
	require.NoError(t, err)
	bJson, err := protojson.Marshal(b)
	require.NoError(t, err)

	if a.EqualVT(b) {
		assert.JSONEq(t, string(aJson), string(bJson))
		err := fmt.Errorf("these %T should not be equal:\nmsg = %+v\noriginal = %+v", a, a, b)
		require.NoError(t, err)
	}
}

This is similar to #48 , but applies to oneofs and exercises different paths in the code generation.

[misberner]

See misberner#2 for the fix (it depends on #49 , so I cannot create a PR against this repo yet, at least not without a blown-up diff)

[vmg]

Oops, closed too soon. Didn't notice this was a separate issue. Can you open a PR for misberner#2? :)

[misberner]

Done, reopened as #51