#303 optional add default to changes

[senid231]

class Account < JsonApiClient::Resource
  property :name
  property :is_active, default: true
  property :balance
end

class User < JsonApiClient::Resource
  self.add_defaults_to_changes = true
  property :name
  property :is_active, default: true
  property :balance
end

account = Account.new
account.changed # []

user = User.new
user.changed # ['is_active']

add optional behavior when default values will be added to payload
related to #303

[gaorlov]

looks good to me