Set attributes in initializer through <attribute>= so it can be overridden w/ a setter.

[bvandenbos]

By settings attributes via method_missing (instead of set_attribute) we can define custom setters in the resource classes and not have those fields hit the server.

class Person < JsonApiClient::Resource
  attr_accessor :my_field
  # ...
end

person = Person.new(name: "Ben", my_field: "Blah")
person.save # => my_field is not passed to the server

Also, adding support for validation contexts: on: :create and on: :update.

class Person < JsonApiClient::Resource
  validates :first_name, presence: true, on: :create
end