fix: sanitize modifier attributes

[ghostwriter]

I used thisdraft.yml file.

models:
   cooltable:
      coolcool: id foreign:coolcool.id index:custom_index_coolcool
      foobar: id foreign:foobars.id index:custom_index_foobar

reverse-engineered from the generated code on #402

Schema::create('cooltable', function (Blueprint $table) {
     19▕             $table->id();
  ➜  20▕             $table->foreignId('coolcool')->constrained()->cascadeOnDelete()->index(custom_index_coolcool);
     21▕             $table->foreignId('foobar')->constrained()->cascadeOnDelete()->index(custom_index_foobar);
     22▕         });

Closes #402

[jasonmccreary]

Thanks @nathane. Email me when you have a minute.