[gatsby-theme-i18n] a wrong md locale node field is created when file extension is .md

[twodft]

Hello, I'm using both .md and .mdx file extension for my blog posts. And while using gatsby-theme-i18n,
I found it generate a wrong locale and isDefault node fields when my blog file extension is setting to .md, the graphql result is like this,

        {
          "node": {
            "fields": {
              "isDefault": false,
              "locale": "md"
            }
          }
        },

after change my blog markdown file extension to .mdx, then I get the correct result.

{
          "node": {
            "fields": {
              "isDefault": true,
              "locale": "en"
            }
          }
        },

[olivier-lacroix]

@twodft , this would be fixed by #130

if a maintainer merges it that is…

[twodft]

fixed by #130