fix: get `User` model when resolving custom user type

[justlevine]

What

Tries to coerce non-core Models into a Model\User when resolving for AuthenticationData

Why

Fixes an oversight in #30 : The WooGraphQL customer model doesn't extend the User, and this generic shim seems like the best approach to supporting other custom types defined by the graphql_login_user_types filter.

How

Testing Instructions

Additional Info

Checklist:

• [x] My code is tested to the best of my abilities.
• [x] My code follows the WordPress Coding Standards.
• [x] My code has proper inline documentation.
• [x] I have added unit tests to verify the code works as intended.
• [x] I included the relevant changes in CHANGELOG.md

[coveralls]

Coverage: 61.707% (-0.08%) from 61.79% when pulling 591d0253d3b102deb8f371bf71a42d606eebf3e5 on justlevine:fix/woo-user-model into da3a33f80f84dd18fdfc1dc388f7be1aaf6d54c0 on AxeWP:develop.