Fix potential ArgumentError in development

When operating in a development environment, changing
Griddler.configuration.processor_class can raise an

    ArgumentError: A copy of EmailProcessor has been removed from the module tree but is still active!

To prevent this, change our reference to the class to a string and
reload it when the class is reloaded (as it is in
Rails.env.development?) and constantize that string when needed.

lib/griddler/configuration.rb

@@ -22,7 +22,7 @@ def processor_class
       @processor_class ||=
         begin
           if Kernel.const_defined?(:EmailProcessor)
-            EmailProcessor
+            "EmailProcessor"
           else
             raise NameError.new(<<-ERROR.strip_heredoc, 'EmailProcessor')
               To use Griddler, you must either define `EmailProcessor` or configure a
@@ -31,6 +31,12 @@ def processor_class
             ERROR
           end
         end
+
+      @processor_class.constantize
+    end
+
+    def processor_class=(klass)
+      @processor_class = klass.to_s
     end
 
     def processor_method

spec/griddler/configuration_spec.rb

@@ -26,13 +26,14 @@
     end
 
     it 'stores a processor_class' do
-      dummy_processor = Class.new
+      class DummyProcessor 
+      end
 
       Griddler.configure do |config|
-        config.processor_class = dummy_processor
+        config.processor_class = DummyProcessor
       end
 
-      expect(Griddler.configuration.processor_class).to eq dummy_processor
+      expect(Griddler.configuration.processor_class).to eq DummyProcessor
     end
 
     it 'stores a processor_method' do