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.

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 });
   }
 }