@Override
    public int getFieldNumber(String name)
    {
        if (name.length() != 1)
            return 0;
        
        char c = name.charAt(0);
        return c == '_' ? ID_POJO : PolymorphicCollectionSchema.number(c);
    }
Will be used if {@link RuntimeEnv#POJO_SCHEMA_ON_COLLECTION_FIELDS} is enabled and the
field is assignable from Collection.

@author David Yu
@created May 19, 2016
protostuff-runtime/src/main/java/io/protostuff/runtime/PolymorphicPojoCollectionSchema.java