fix frontmatter stringification for date fields

Switches back to custom frontmatter stringification
until support lands in preliminaries. This is necessary
because we use custom schemas for certain data types,
such as dates and times.

example/config.yml

@@ -12,10 +12,10 @@ collections: # A list of collections the CMS should be able to edit
     create: true # Allow users to create new documents in this collection
     fields: # The fields each document in this collection have
       - {label: "Title", name: "title", widget: "string", tagname: "h1"}
+      - {label: "Publish Date", name: "date", widget: "datetime", format: "YYYY-MM-DD hh:mma"}
       - {label: "Cover Image", name: "image", widget: "image", required: false, tagname: ""}
       - {label: "Body", name: "body", widget: "markdown"}
     meta:
-      - {label: "Publish Date", name: "date", widget: "datetime", format: "YYYY-MM-DD hh:mma"}
       - {label: "SEO Description", name: "description", widget: "text"}
     card: {type: "image", image: "image", text: "title"}
 

src/formats/frontmatter.js

@@ -1,6 +1,7 @@
 import preliminaries from 'preliminaries';
 import yamlParser from 'preliminaries-parser-yaml';
 import tomlParser from 'preliminaries-parser-toml';
+import YAML from './yaml';
 
 // Automatically register parsers
 preliminaries(true);
@@ -25,7 +26,13 @@ export default class Frontmatter {
         meta[key] = data[key];
       }
     });
+
     // always stringify to YAML
-    return preliminaries.stringify(body, meta, { lang: 'yaml', delims: '---' });
+    const parser = {
+      stringify(metadata) {
+        return new YAML().toFile(metadata, sortedKeys);
+      },
+    };
+    return preliminaries.stringify(body, meta, { lang: "yaml", delims: "---", parser });
   }
 }