fix frontmatter stringification for date fields

[erquhart]

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

- Summary

Date fields can no longer be changed through the CMS. Regression from #348.

- Test plan

Moved the Published Date field in the example project out of metadata and into standard fields, so persisting dates can be observed in deploy previews. Tested locally.

- Description for the changelog

Fix date/datetime field persisting

[Benaiah]

LGTM and fixes the issue on my test repo.

EDIT: looks like a test needs to be updated; the Travis build is failing.

[josephearl]

👍 good idea

[josephearl]

Another option is to pass a custom parser implementation that wraps the existing YAML to preliminaries:

var parser = {
  stringify: function(meta) {
    return new YAML().toFile(meta, sortedKeys)
  }
}
return preliminaries.stringify(body, meta, { lang: 'yaml', delims: '---', parser });

[calavera]

I like @josephearl's suggestion.

[Benaiah]

@erquhart @calavera @josephearl I have the tests passing (mocked src/valueObjects/AssetProxy.js and updated a stringify test which had slightly different whitespace). Once the test changes are reviewed it should be ready to merge.

[calavera]

Thanks @Benaiah.

LGTM

[erquhart]

@josephearl can you update to use our custom parser with preliminaries? Otherwise I will when I get back online.

[Benaiah]

Closing in favor of #385, which uses the implementation suggested by @josephearl.