Mail does not display body of plain text email

There seems to be something wrong when Mail parses plain text emails.
```
ralph@artful:~/git/mail/build$ io.elementary.mail
** (io.elementary.mail:5829): CRITICAL **: mail_web_view_add_internal_resource: assertion 'name != NULL' failed
** (io.elementary.mail:5829): CRITICAL **: mail_web_view_add_internal_resource: assertion 'name != NULL' failed
** (io.elementary.mail:5829): CRITICAL **: void webkit_web_view_load_plain_text(WebKitWebView*, const gchar*): assertion 'plainText' failed
```
Example email [attached](https://github.com/elementary/mail/files/1362561/Bits.from.the.DPL.September.2017.zip).

---
Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/56465046-mail-does-not-display-body-of-plain-text-email?utm_campaign=plugin&utm_content=tracker%2F59850425&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F59850425&utm_medium=issues&utm_source=github).