Add support for single record serialization

[felixbuenemann]

This will handle serialization of single records as used for example in show actions using the postres array serializer. To be able to do this, the record is wrapped in a relation using its primary key for lookup.

It's also possible to use this feature with a relation by passing the root: 'singular_name' and single_record: true options to render or the serializer, but you have to ensure that the relation only returns a single record, for example by using .limit(1).

The single record serialization format does use a singular root key name for the main resource and does not wrap the object in an array:

Collection: {"notes":[{"id":1,"tag_ids":[1]}],"tags":[...]}
Single Record: {"note":{"id":1,"tag_ids":[1]},"tags":[...]}

The latter format is required by ember-data with the active model adapter when using find('note', 1).

This PR does not add support for the root: false option (see #13). For single records a fallback to the regular serializer will be used when root is disabled.

[felixbuenemann]

Currently this PR switches the default serializer for single records from ruby to postgres and is likely to break apps that currently us a mix of ruby and postgres serialization, if the resources currently serialized by ruby are not fully compatible with postgres_ext-serializers.

As such merging this PR should require a major version bump.

Another option would be to make the :single_record option opt-in and change it's default when releasing the next version with breaking changes.

[felixbuenemann]

I think this is, because it's not actually using the database serialization. We should alter the test to ensure the original serializers are not called.

[felixbuenemann]

This needs some more work. The "serialize singular record" test is not working as intended.

Currently we need to manually pass a record to the ArraySerializer to be able to use database serialization. In order for this to happen for render json: record we need to alter the ActiveModel::Serializer.build_json class method to pass the record to the ArraySerializer instead of the default serializer.

[felixbuenemann]

I've revised the code and moved the relation wrapping into a patch to ActiveModel::Serializer.build_json which is cleaner and fixes automatic database serialization when passing records to render :json.

I've also extended the tests to make sure the ArraySerializer is used for serialization and that using custom serializers works as expected.

[felixbuenemann]

Rebased on current master.

[felixbuenemann]

Instead of adding an option to enable/disable single record database serialization, there should probably be an option like use_postgres_serializer: false to disable postgres_ext-serializers on a case by case basis, eg. using default_serializer_options in the controller, or an option passed to render :json.

This would make it easier to work with projects where not all serializers are compatible with postgres_ext-serializers and ease migration towards full database serialization.

[felixbuenemann]

I'm going to merge this as is and add the option to conditionally disable database serialization in a separate PR. This means the next release should be 1.0.0.