Fix DEPRECATION WARNING: ActiveModel::Errors#values is deprecated

[jrhoads]

Purpose

Fix the deprication warning.

closes: #datacite/datacite/issues/991

Approach

The original deprication warning included the fix. This just implemented it.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

• New feature (non-breaking change which adds functionality)

• Breaking change (fix or feature that would cause existing functionality to change)

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