Only set first name to the users first name or display name depending…

[kimcoleman]

… on what we have available

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Changes proposed in this Pull Request:

We should only set the subscriber's "First Name" to their user First Name (if available). Since a name is required in the API, we will fall back to display_name since that is always set on a WP user.

Closes Issue: #28

Changelog entry

• BUG FIX/ENHANCEMENT: Updated logic for $user_name to use the first name if available, otherwise falling back to the display name.