`SeparetorError` fix messes up frontmatter

[twanh]

When there is frontmatter the --- is seen as a separator and a new blank line is added after the ---.

E.g.:

---
title: Test test
---

get changed to:

---

title: Test test
---

In frontmatter this should not happen since it makes the frontmatter invalid.

A possible solution is to check if the --- separator is on the first line of the file and in that case skip it.