Fix conditional procedures with lambdas - #59

What is the current behavior?

Conditional procedures for scalars and relationships don't work when used with lambdas, for example:

class MovieSerializer
  include FastJsonapi::ObjectSerializer

  attribute :release_year, if: ->(record) { record.release_year >= 2000 }
  has_many :actors, if: ->(record) { record.actors.any? }
end

These currently throw an error: ArgumentError: wrong number of arguments (given 2, expected 1).

This is very similar to the &:proc shorthand issue I've been working on at #58.

What is the new behavior?

This change fixes the behavior to be able to use lambdas as well as Procs with either one or two parameters.

This also includes the arity check for lambdas, which could be extracted to a common place.

Checklist

Please make sure the following requirements are complete:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes /
  features)
• All automated checks pass (CI/CD)