Have attributes_builder handle MIQ virtual_attributes

This overwrites the default method for this by adding an additional
check to the proc that is defined with the AttributeSet::Builder to
check to see if the key should be provided a default from the
`_default_attributes` hash to not only check to see if it is a column
from the database, but also a virtual_attribute.

The functionality added by:

rails/rails@3e37a8b#diff-6c8fd220c92c2b25aa29891b59d00c04

was meant to help vanilla Rails' "virtual attributes" by assigning a
default value on model instantiation.  This is not what used to be done
for any attributes (database or otherwise) besides the primary key for
that model.  What this does is includes `virtual_attributes` as
attributes that aren't defined with a default value, along the database
column attributes.

lib/extensions/ar_virtual.rb

@@ -386,6 +386,14 @@ def virtual_attribute_names
       end
     end
 
+    def attributes_builder
+      @attributes_builder ||= ::ActiveRecord::AttributeSet::Builder.new(attribute_types, primary_key) do |name|
+        unless columns_hash.key?(name) || virtual_attribute?(name)
+          _default_attributes[name].dup
+        end
+      end
+    end
+
     private
 
     def load_schema!