allow passing proc for relationships with id_method_name

[rishabhsairawat]

When using id_method_name in relationship, we can allow passing simple proc object shortcut to relationship in place of a block.
Fixes #399
For example:

 class User
  include FastJsonapi::ObjectSerializer

  set_id :uuid
  set_key_transform :dash
  set_type :users 
  attributes :name, :email 

 # To support this (currently results into error)
  belongs_to :organization, record_type: :organizations, id_method_name: :uuid, &:organization


 # Instead of this
 # belongs_to :organization, record_type: :organizations, id_method_name: :uuid do |object|
 #   object.organization
 # end
end