#299 optional get relationship from dataset

[senid231]

related to #299

simple example

class Employee < JsonApiClient::Resource
  self.search_included_in_result_set = true
  has_one :chief, klass: 'Employee'
end

stub_request(:get, '/employees?include=chief') do
{ data: [
    {
        id: '1', 
        type: 'employees', 
        attributes: {  name: 'John' },
        relationships: {
            chief: {
                data: null
            }
        }
    },
    {
        id: '2', 
        type: 'employees', 
        attributes: {  name: 'Bob' },
        relationships: {
            chief: {
                data: { id: '1', type: 'employees' }
            }
        }
    }
] }.to_json
end
employees = Employee.includes(:chief) # responds with
bob = employees.detect { |e| e.name == 'Bob' }
assert_equal 'John', bob.chief.name