Fix setting connection_specification_name on ActiveRecord::Base

Since 2f8b397258b66581409b0e6537f98ea9b56e9f19, `ActiveRecord::Base` and
`ApplicationRecord` use the same default `connection_specification_name`
so that database connections configured on `ApplicationRecord` also
apply to `ActiveRecord::Base`.

However, when resolving the connection for `ApplicationRecord` we should
continue to fall back to `ActiveRecord::Base` when there's no connection
explicitly configured, so that setting `connection_specification_name`
on `ActiveRecord::Base` affects `ApplicationRecord` and its descendants
in this case as it did in previous versions.

activerecord/lib/active_record/connection_handling.rb

@@ -204,7 +204,7 @@ def connection
     # Return the specification name from the current class or its parent.
     def connection_specification_name
       if !defined?(@connection_specification_name) || @connection_specification_name.nil?
-        return primary_class? ? "primary" : superclass.connection_specification_name
+        return self == Base ? "primary" : superclass.connection_specification_name
       end
       @connection_specification_name
     end

activerecord/test/cases/connection_adapters/connection_handler_test.rb

@@ -375,6 +375,8 @@ class MyClass < ApplicationRecord
         end
 
         def test_connection_specification_name_should_fallback_to_parent
+          Object.send :const_set, :ApplicationRecord, ApplicationRecord
+
           klassA = Class.new(Base)
           klassB = Class.new(klassA)
           klassC = Class.new(MyClass)
@@ -387,6 +389,12 @@ def test_connection_specification_name_should_fallback_to_parent
 
           klassA.connection_specification_name = "readonly"
           assert_equal "readonly", klassB.connection_specification_name
+
+          ActiveRecord::Base.connection_specification_name = "readonly"
+          assert_equal "readonly", klassC.connection_specification_name
+        ensure
+          Object.send :remove_const, :ApplicationRecord
+          ActiveRecord::Base.connection_specification_name = "primary"
         end
 
         def test_remove_connection_should_not_remove_parent