Correct domain replacement for GraphQL fields

[apmatthews]

Resolves #541

As mentioned in #541, it appeared we were not doing domain replacement on menu item URLs. Upon investigation, it appears we are performing replacements that should cover menu item URLs, but these replacements were short-circuited by the presence of generalSettings in the response data. I believe the intention here was to skip replacements inside of generalSettings, as queries for generalSettings.url should accurately report the URL of the WordPress instance rather than the front-end application.

With the way GQty assembles queries, it's likely generalSettings will be included in most requests. This PR changes the replacement callback to skip generalSettings without preventing replacements elsewhere in the response data.

Testing

• Unit tests have been added.

Manual Testing

1. Before pulling this branch, make a simple menu in Appearance->Menus and assign it to the Primary menu location.
2. Run the following query in GraphiQL:

query MyQuery {
  generalSettings {
    url
  }
  menuItems(where: {location: PRIMARY}) {
    nodes {
      label
      url
    }
  }
  menus(where: {location: PRIMARY}) {
    nodes {
      menuItems {
        nodes {
          label
          url
        }
      }
    }
  }
}

3. See that no url fields receive domain replacements
4. Pull the branch
5. Run the same query as above
6. See that generalSettings.url is unchanged, but all menu item urls have been rewritten:

{
  "data": {
    "generalSettings": {
      "url": "http://faustjs.local"
    },
    "menuItems": {
      "nodes": [
        {
          "label": "Hello World",
          "url": "http://localhost:3000/hello-world/"
        }
      ]
    },
    "menus": {
      "nodes": [
        {
          "menuItems": {
            "nodes": [
              {
                "label": "Hello World",
                "url": "http://localhost:3000/hello-world/"
              }
            ]
          }
        }
      ]
    }
  }
}

[blakewilson]

LGTM 👍