Cannot save any data files after new markdown editor.

[tech4him1]

- Do you want to request a feature or report a bug?
bug

- What is the current behavior?
After #254 (the new editor) was added, any data file on the example site (test-repo backend), whether it contains a markdown field or not, can no longer be saved. The console error is:

Uncaught TypeError: Cannot read property 'reduce' of undefined
    at runSerializer (serializeEntryValues.js?6de9:31)
    at serializeValues (serializeEntryValues.js?6de9:62)
    at eval (entries.js?c1a9:279)
    at eval (index.js?8a13:9)
    at eval (bindActionCreators.js?b33a:3)
    at eval (EntryPage.js?2e7b:100)

Related to #575 (cc/@erquhart).
- If the current behavior is a bug, please provide the steps to reproduce.

1. Run the example site, or any site with a config with a data file like this: https://github.com/netlify/netlify-cms/blob/6af9a3155c794cec697bc0722907c738200e292b/example/config.yml#L29-L46.
2. Open the CMS and edit the data file (settings.json in the example).
3. Try to save the entry.

- What is the expected behavior?
The entry should be saved.

[tech4him1]

Tracing this back, at this line collection.get('fields') is resolving to undefined:
https://github.com/netlify/netlify-cms/blob/ac8df984074fec231a470ad2aaca5fe3299049f0/src/actions/entries.js#L279

I'm thinking this is because data file collections, instead of having fields directly, have files which then have their own fields. The interesting thing is that as far as I know this worked before, so I'm not exactly sure what changed.