MessageDyn: Do not omit zero values on fields containing oneof

[mingyuchi]

Omitting zeros on singular fields only make sense if it's not in oneof
For example, omitting zero on the following oneof will make it ambiguous

message IntOrString {
  oneof int_or_string {
    int32 int = 1;
    string str = 2;
  }
}

if either int or str is zero, both of the fields will be missing.
Now it's not possible to know which field was originally filled

[SeongMinPark-devsisters]

@stepancheg Hello, what do you think of this pr?