Allow association definition to specify serializer

[midu]

Found myself trying to find out a way to specify associations on the serializer level, and realized it wasn't supported. Here's a first pass at implementing that functionality.

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For some extra background, my use case was the following:

I have a resource that references objects of the same class OR subclasses. But for the serialization, I want them to be all serialized the same way:

Something along these lines

# Directory has several subclasses, but when I serialize a directory, I want all `parent_directory` and `child_directories` to be serialized as if they were just a regular "directory"
class Directory
  attr_accessor :parent_directory
  attr_accessor :child_directories
  attr_accessor :parition
end

class Parition < Directory
end

And this is what I wanted to be able to do:

class DirectorySerializer
  has_one :parent_directory, serializer: DirectorySerializer
  has_many :child_directories, serializer: DirectorySerializer
  has_one :partition, serializer: PartitionSerializer 
end

The changes in this PR enable this kind of configuration.

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Looking through the code, I realized that we're always passing the options passed to the serializer for the main resource to all the included associations. Instead with this PR, we are passing the association options.

There was one main issue highlighted by the tests, and I am not sure that my fix is in line with what the maintainers of this project want. Basically, if a resource is serialized twice (as the main resource and in the under the included key), with my change the one in the included resource doesn't get the customization options from the serialization of the root resource.

I think it makes sense. Please let me know if you disagree.

I would actually argue that it would be safer to require each association to explicitly specify the serializer instead of having the serializer infer from the object class. But that may not be a very popular opinion.

[midu]

Just realizing it does the same as #94

feel free to close if you prefer #94