Call constantize on string prior to .find call (#192)

## Changes

Somehow these changes were made without the tests failing, but we are
passing a string back as the class. We need to return it as a Ruby
constant class name in order to call methods on it.

Author: baonguyenNava

app/models/flex/case.rb

@@ -38,7 +38,7 @@ def self.business_process
     # Returns the application form class for this case class.
     # Subclasses can override this method to customize the application form class naming.
     def self.application_form_class
-      name.sub("Case", "ApplicationForm")
+      name.sub("Case", "ApplicationForm").constantize
     end
     attribute :application_form_id, :uuid