Suppress Ruby 2.7 kwargs warnings
Suppress Ruby 2.7 kwargs warnings

I've suppressed Ruby 2.7 kwargs warnings.

And I made belongs_to method the same interface to ActiveRecord one,
because it uses "super" inside its implementation.

If this pull request merged, these warnings are going to be suppressed.

```
/path/to/ruby/2.7.0/gems/active_hash-3.1.0/lib/associations/associations.rb:22: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
```

zilkey/active_hash,kbrock/active_hash