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

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 "testspace.md" with slugSeparator is set to "".

Mac OS X: 12.3 Atom: 1.60.0

To Reproduce Steps to reproduce the behavior:

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
```
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.
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)
```
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)
```
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:

zhuochun / md-writer

{slug} does not replace space with slugSeparator used with siteImagesDir #312