Fix: Extensions aren't loaded in development/test

PR #19 previously loaded the associated object class when the has_object
was defined, so that extensions would get run. It seems like that code
got lost in a rebase. So now, extensions only run on boot if eager
loading is enabled (which it is in production). Otherwise they aren't
loaded until the first time the associated object gets used.

lib/active_record/associated_object/object_association.rb

@@ -14,7 +14,7 @@ def extend_source_from(chunks, &block)
   module ClassMethods
     def has_object(*names, **callbacks)
       extend_source_from(names) do |name|
-        "def #{name}; (@associated_objects ||= {})[:#{name}] ||= #{name.to_s.classify}.new(self); end"
+        "def #{name}; (@associated_objects ||= {})[:#{name}] ||= #{const_get(name.to_s.classify)}.new(self); end"
       end
 
       extend_source_from(names) do |name|