[gatsby-source-contentful] Added support for non localized fields without use of fallback locales

[stoltzrobin]

Fixes for #2937, default locale is added to support a contentful setup where fallback locales are not used.

Test case

A content model existing of 2 fields (the environment is setup without having any fallbacks.

• Name (localized)
• slug (not localized)

Result from graphQL query

{
  "data": {
    "allContentfulPage": {
      "edges": [
        {
          "node": {
            "name": "Page A",
            "node_locale": "en-US",
            "slug": "/"
          }
        },
        {
          "node": {
            "name": "Page B",
            "node_locale": "es-ES",
            "slug": null
          }
        },
      ]
    }
  }
}

Expected Result

{
  "data": {
    "allContentfulPage": {
      "edges": [
        {
          "node": {
            "name": "Page A",
            "node_locale": "en-US",
            "slug": "/"
          }
        },
        {
          "node": {
            "name": "Page B",
            "node_locale": "es-ES",
            "slug": "/"
          }
        },
      ]
    }
  }
}

[pieh]

@Khaledgarbaya is this change a valid behaviour change? I don't know enough about contentful localization setup to make that call.

[Khaledgarbaya]

Hey @stoltzrobin,
can you add some test so I get to see the problem better?

Best,
Khaled

[Khaledgarbaya]

LGTM

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