handle models that are not instances of the Model class

[macbookandrew]

Fixes #202

I’m using the woocommerce plugin and getting the same error mentioned in #202. Rather than an instance of \WPGraphQL\Model\Model, the plugin is receiving an array that looks like this:

{
    "download_url": "https:\/\/www.example.com\/?download_file=3725&order=wc_order_EYw4cHxUlpAsV&email=john.ode%40example.com&key=fa5895b4-16aa-4826-8b20-ca8fcfbdb857",
    "download_id": "fa5895b4-16aa-4826-8b20-ca8fcfbdb857",
    "product_id": 3725,
    "product_name": "Example Product - Audio",
    "product_url": "https:\/\/www.example.com\/product\/example-product\/?attribute_pa_download-options=audio",
    "download_name": "Example Product (audiobook .zip file)",
    "order_id": 10501,
    "order_key": "wc_order_EYw4cHxUlpAsV",
    "downloads_remaining": "",
    "access_expires": null,
    "file": {
        "name": "Example Product (audiobook .zip file)",
        "file": "https:\/\/example.s3.us-east-2.amazonaws.com\/pda\/www.example.com\/wp-content\/uploads\/woocommerce_uploads\/example-product-audiobook.zip?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=…"
    }
}

Since we can’t reliably infer the model name or database ID for any possible array that the plugin might receive, I figured it was probably best to bypass creating surrogate keys.

[jspellman814]

Thanks for your contribution! I copied this to #212 and am going to close this PR and continue the conversation there.