Bug: File Transclusion will strip away the first line if there's some spaces

Here is a MWE showing the bug: [temp.zip](https://github.com/fletcher/MultiMarkdown-5/files/203578/temp.zip)

In the `original` folder, things are working as expected.

In the `bug` folder, in file `ch1.md`, `ch2.md`, I put 2 spaces in the line between the metadata front matter and the first header. In the transcluded file `master.html`, you will see that the header of `ch1.md`, `ch2.md` has gone.

Start by extracting temp.zip and comparing the original and bug folders, especially ch1.md, ch2.md, and the generated master.html. Trace the file-transclusion path that handles the blank line after metadata front matter. Done means the headers in both transcluded files remain present when that line contains two spaces.

Written by the indexing model from the issue text.