[Fix #603] Teach RedundantPresenceValidationOnBelongsTo to work with multi-attribute validations

[pirj]

fixes #603

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[koic]

Although not directly related to this PR change, can you suppress the following yard doc warning?

% bundle exec rake
(snip)

[warn]: @param tag has unknown parameter name: key
    in file `lib/rubocop/cop/rails/redundant_presence_validation_on_belongs_to.rb' near line 133

[pirj]

@param tag has unknown parameter name: key

Thanks for noticing, this warning evaded me. Fixed.

[koic]

Thanks!