{slug} does not replace space with slugSeparator used with siteImagesDir

[fkjensen]

Describe the bug
{slug} does not replace space with slugSeparator used with siteImagesDir.

I created a project specific mdwriter.cson configuration with a minimal setup where I would like the markdown filename to be used with the {slug} counterpart. I expected that the filename "test space.md" would have a corresponding {slug} as "test_space.md" with slugSeparator is set to "".

Mac OS X: 12.3
Atom: 1.60.0

To Reproduce
Steps to reproduce the behavior:

1. First I do a new project "wr" by making a directory named "t/wr". In that directory I use Atom to create a new "_mdwriter.cson" project specific configuration file with the following configuration

siteEngine: "general"
fileExtension: ".md"
slugSeparator: "_"
siteLocalDir: ""
siteImagesDir: "assets/{slug}/"
imageTag: "![{alt}]({relativeSiteSrc})"
relativeImagePath: true

2. On Mac OS X based Atom I do a "Cmd-Shift-3" to copy a screenshot and then do a "Cmd-Shift-I" to copy the screenshot into a project specific folder configured.

3. With a filename "testnospace.md" I happen to get the following link and a file in assets/testnospace/a.png as intended and expected

# Test No Space In Filename
![a](assets/testnospace/a.png)

4. With a filename "test space.md" I happen to get the follwing link and a file in "assets/test space/a.png" which is not what I intended or expected. I know that I expect {slug} to be computed from the filename.

test one:
![a](assets/test space/a.png)

5. I tried to set the slug manually by adding the slug defintion to frontmatter:

---
slug: test_space
---
test one:
![a](assets/test space/a.png)

test two with manual slug in frontmatter:
![b](assets/test space/b.png)

Expected behavior
I expected the following content after doing the clipboard screenshot and a paste of the clipboard to Atom. {slug} with filename spaces replaced by slugSeparator is expected. Am I missing something here or is it a bug?

test one:
![c](assets/test_space/c.png)

[zhuochun]

Currently, filename is assumed to be slug-ed and used directly to {slug} placeholder.

But agree, front matter slug should be respected.