Prevent ActiveRecord from reloading associated records when the STI name is different

[mctaylorpants]

⚠️ Depends on #25
⚠️ Depends on #26

---

When using classes with custom STI names, a bug exists where ActiveRecord will always reload the associated record from the database, even if a valid instance is already loaded.

This occurs because of how we are patching foreign_type=:

define_method "#{foreign_type}=" do |klass|
  mapping = self.class.send("#{foreign_type}_mapping")
  enum = mapping.key(klass.to_s)
  if klass.kind_of?(Class) && klass <= ActiveRecord::Base
    enum ||= mapping.key(klass.sti_name)
    enum ||= mapping.key(klass.base_class.to_s)
    enum ||= mapping.key(klass.base_class.sti_name)
  end
end

We expect that klass will be something that we can look up in the foreign type mapping; if we can't look it up, and it's a constant, then we can attempt to find it using its sti_name.

However, sometimes Rails passes this method a string instead of a constant. When that happens, our lookup doesn't work and we end up returning nil. As a result, the associated record is reloaded because ActiveRecord believes it to be stale.

This issue only exists in Rails < 5.2. Rails 5.2 introduced a .polymorphic_name method that allows the user to customize this as needed.