Fix: Live Preview Update Breaks Frontmatter if Frontmatter Exists and Update Happens on the --- of the end of the Frontmatter

[pjkaufman]

Fixes #1093

There was an issue in Live Preview mode where sometimes updating the value of the frontmatter would cause the frontmatter to become broken. What was happening was that when the frontmatter existed, if you updated the very end of the frontmatter, it was not accounting for the ending --- of the frontmatter in what to ignore on the updates. Thus it would attempt a partial update which would mess up the document.

Changes Made:

• Add an offset to ignore the ending --- of the frontmatter
• Added an integration test for this scenario