Add more tests to embed option

[arthurnn]

• More tests to embed in class level Serializer.
• Second test is marked as skip because is falling as we need to fix the SETTINGS constant on the Serializer.

EDIT
Removing SETTINGS global variable, and make it a class level settings.

review @spastorino

[coveralls]

Coverage decreased (-0.65%) when pulling 4c0b454 on arthurnn:embed_tests into c65d387 on rails-api:master.

[coveralls]

Coverage decreased (-0.01%) when pulling 4bea0c9 on arthurnn:embed_tests into c65d387 on rails-api:master.

[arthurnn]

After talking to @spastorino, we decide to close this, as the idea is to deprecate embed method on a pre-class basis, and make it global. Something like this:

ActiveModel::Serializer.setup do |settings|
  settings[:embed] = :ids
  settings[:include] = true
end