Fixed grouped checkboxes repopulating, changed syntax for repopulating groupboxes

This pull request needs discussed before merging. It has potential for breaking existing code (though any code trying to repopulate grouped checkboxes is already broken).

This patch does two things:
1. Fixes repopulating of grouped checkboxes. The reason this wasn't possible before is that repopulating was done only by the "name" attribute, but grouped checkboxes can have identical names. My proposed fix is to also allow repopulating checkboxes by the "id" attribute.
2. Changes the default value for grouped checkboxes to always be "1", like other checkboxes. In practice in PHP, having a checkbox with a value of 0 is a recipe for confusion and problems. It seems much cleaner to default all values to 1, and allow overriding of values via Former's current syntax: `Former::checkboxes('foo')->grouped()->checkboxes('Value 01' => ['id' => 'my_id', 'value' => 'my_value'])`.

I am glad to separate these into two separate issues if there is debate on one but not the other.