Automatically pick up presence validators for default belongs_to association

[donv]

Automatically pick up presence validators for an association for a given ID attribute.

This enables form validation with messages for default belongs_to associations since Rails 5.

[lcreid]

This is a great find, and a nice solution. I'm hoping it's not too hard to make it more general, for when the foreign key column name doesn't match Rails' conventions.

It would be nice to see a test to show what the new code does.

[UweKubosch]

I'll do at least an entry in the README to see what the code does.

[lcreid]

I won't merge this in case you want to do the README in this PR. Otherwise, you can merge this and do another PR.

[donv]

Hey good people! I added the fully generalized version and added a section in the README displaying the resulting code.

Please have a look. Squash and merge if it looks good.

[donv]

Oh, wait! One more fix needed!

[donv]

Now, go!