datetime and time types should still use nil adapter

Author: jNicker

lib/attr_json/attribute_definition.rb

@@ -130,7 +130,7 @@ def self.lookup_type(type)
         # serializing, we're kind of stuck with this precision in current implementation.
         lookup_kwargs = { adapter: nil }
         if type == :datetime || type == :time
-          lookup_kwargs = { precision: ActiveSupport::JSON::Encoding.time_precision }
+          lookup_kwargs[:precision] = ActiveSupport::JSON::Encoding.time_precision
         end
 
         type = ActiveRecord::Type.lookup(type, **lookup_kwargs)