Use the DefaultSerializer for embedded objects when no serializer is specified

[plexus]

When using embed :ids, include: true, and the model class being embedded does
not implement active_model_serializer, then a default serialization should
happen.

The else branch in ActiveModel::Serializer::Association#find_serializable did
not yet have a test case, and would cause the following error :

 NoMethodError: undefined method `object' for #<Model:0x123>
    active_model_serializers/lib/active_model/serializer.rb:411:in `block in merge_association'

This is because merge_association calls find_serializable, and expects an
object that responds to #object and #serializable_hash, which is not the
case for the model instance itself. This commit wraps the object in a
DefaultSerializer so it satisfies the interface the caller expects.

[coveralls]

Coverage increased (+0.07%) when pulling 6e81363 on plexus:default_serializer_for_embed_include into 919bb38 on rails-api:master.

[agrobbin]

I ran into this problem as well when attempting to serialize an Array of Structs (spoofing a has_many relationship). Would be great to get this merged!

[spastorino]

Current master does exactly that. Check it out and if you need something else, just let me know.
Thank for working on this anyway.