Allow the use of callable objects as group methods for grouped selects.

[Aquaj]

The option_groups_from_collection_for_select helper could only take method names as group_method and group_label_method parameters. This wasn't consistent with option_from_collection_for_select which could take either method names or callable objects.

This PR replaces the uses of group.send(...) in the helper by calls to value_for_collection(group, ...), allowing the use of procs, lambdas and other callable objects as parameters.

[rails-bot]

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[georgeclaghorn]

Seems legit. Thanks!

[Aquaj]

My pleasure 😄