@Override
            @SuppressWarnings("unchecked")
            public void mergeFrom(Input input, Object owner) throws IOException
            {
                if (PMapWrapper.class == owner.getClass())
                {
                    // called from ENTRY_SCHEMA
                    ((PMapWrapper) owner).setValue(ObjectSchema.readObjectFrom(
                            input, this, owner, IdStrategy.this));
                }
                else
                {
                    // called from COLLECTION_SCHEMA
                    ((Collection<Object>) owner).add(ObjectSchema.readObjectFrom(
                            input, this, owner, IdStrategy.this));
                }
            }
This base class handles all the IO for reading and writing polymorphic fields. When a field's type is
polymorphic/dynamic (e.g interface/abstract/object), the type (id) needs to be written (ahead) before its
value/content to be able to deserialize it correctly.
<p>
The underlying impl will determine how the type (id) should be written.
<p>
An {@link IdStrategy} is standalone if the {@link #primaryGroup} is not set.

@author Leo Romanoff
@author David Yu
protostuff-runtime/src/main/java/io/protostuff/runtime/IdStrategy.java