title: Error when create a new Calculator on Spree Commerce gem
body: <p>I am learning rails and I like to learn by practicing trying to create something real.</p>
  
  <p> I'm having some problems trying to create a new calculator to be used in the shipping area of ​​spree commerce.</p>
  
  <p>Following this documentation tutorial.</p>
  
  <p><a href="http://guides.spreecommerce.org/developer/calculators.html#creating-a-new-calculator" rel="nofollow noreferrer">http://guides.spreecommerce.org/developer/calculators.html#creating-a-new-calculator</a></p>
  
  <p>Following the tutorial i created this class in this directory.</p>
  
  <pre><code>app/models/spree/calculator/shipping/my_own_calculator.rb
  </code></pre>
  
  <p>My class is this way</p>
  
  <pre><code>class MyOwnCalculator &lt; Spree::ShippingCalculator
    def self.description
      Spree.t(:shipping_flat_rate_per_order)
    end
  
    def compute_package(package)
      self.preferred_amount
    end
  
    def available?(object)
      object.currency == "USD"
    end
  end
  </code></pre>
  
  <p>and my file <code>config/initializers/spree.rb</code></p>
  
  <pre><code># Configure Spree Preferences
  #
  # Note: Initializing preferences available within the Admin will overwrite any changes that were made through the user interface when you restart.
  #       If you would like users to be able to update a setting with the Admin it should NOT be set here.
  #
  # Note: If a preference is set here it will be stored within the cache &amp; database upon initialization.
  #       Just removing an entry from this initializer will not make the preference value go away.
  #       Instead you must either set a new value or remove entry, clear cache, and remove database entry.
  #
  # In order to initialize a setting do:
  # config.setting_name = 'new value'
  Spree.config do |config|
    # Example:
    # Uncomment to stop tracking inventory levels in the application
    # config.track_inventory_levels = false
  end
  
  Spree.user_class = "Spree::User"
  
  config = Rails.application.config
  config.spree.calculators.shipping_methods &lt;&lt; Spree::Calculator::Shipping::MyOwnCalculator
  </code></pre>
  
  <p>the error is this: </p>
  
  <pre><code>Unable to autoload constant Spree::Calculator::Shipping::MyOwnCalculator, expected /home/davi/dev/attsalaoruby/app/models/spree/calculator/shipping/my_own_calculator.rb to define it (LoadError)
  </code></pre>
  
  <p>and my stack trace is this way </p>
  
  <pre><code>/.rvm/gems/ruby-2.3.0@5.0.0.1/gems/activesupport-5.0.1/lib/active_support/dependencies.rb:512:in `load_missing_constant': Unable to autoload constant Spree::Calculator::Shipping::MyOwnCalculator, expected /home/davi/dev/attsalaoruby/app/models/spree/calculator/shipping/my_own_calculator.rb to define it (LoadError)
      from /home/davi/.rvm/gems/ruby-2.3.0@5.0.0.1/gems/activesupport-5.0.1/lib/active_support/dependencies.rb:203:in `const_missing'
      from /home/davi/dev/attsalaoruby/config/initializers/spree.rb:21:in `&lt;top (required)&gt;'
      from /home/davi/.rvm/gems/ruby-2.3.0@5.0.0.1/gems/activesupport-5.0.1/lib/active_support/dependencies.rb:287:in `load'
      from /home/davi/.rvm/gems/ruby-2.3.0@5.0.0.1/gems/activesupport-5.0.1/lib/active_support/dependencies.rb:287:in `block in load'
      from /home/davi/.rvm/gems/ruby-2.3.0@5.0.0.1/gems/activesupport-5.0.1/lib/active_support/dependencies.rb:259:in `load_dependency'
      from /home/davi/.rvm/gems/ruby-2.3.0@5.0.0.1/gems/activesupport-5.0.1/lib/active_support/dependencies.rb:287:in `load'
      from /home/davi/.rvm/gems/ruby-2.3.0@5.0.0.1/gems/railties-5.0.1/lib/rails/engine.rb:648:in `block in load_config_initializer'
      from /home/davi/.rvm/gems/ruby-2.3.0@5.0.0.1/gems/activesupport-5.0.1/lib/active_support/notifications.rb:166:in `instrument'
      from /home/davi/.rvm/gems/ruby-2.3.0@5.0.0.1/gems/railties-5.0.1/lib/rails/engine.rb:647:in `load_config_initializer'
      from /home/davi/.rvm/gems/ruby-2.3.0@5.0.0.1/gems/railties-5.0.1/lib/rails/engine.rb:612:in `block (2 levels) in &lt;class:Engine&gt;'
      from /home/davi/.rvm/gems/ruby-2.3.0@5.0.0.1/gems/railties-5.0.1/lib/rails/engine.rb:611:in `each'
      from /home/davi/.rvm/gems/ruby-2.3.0@5.0.0.1/gems/railties-5.0.1/lib/rails/engine.rb:611:in `block in &lt;class:Engine&gt;'
      from /home/davi/.rvm/gems/ruby-2.3.0@5.0.0.1/gems/railties-5.0.1/lib/rails/initializable.rb:30:in `instance_exec'
      from /home/davi/.rvm/gems/ruby-2.3.0@5.0.0.1/gems/railties-5.0.1/lib/rails/initializable.rb:30:in `run'
      from /home/davi/.rvm/gems/ruby-2.3.0@5.0.0.1/gems/railties-5.0.1/lib/rails/initializable.rb:55:in `block in run_initializers'
      from /home/davi/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/tsort.rb:228:in `block in tsort_each'
      from /home/davi/.rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/tsort.rb:350:in `block (2 levels) in each_strongly_connected_component'
  ....
  </code></pre>