concat directory separators instead of "/" to make it work on Windows…

[Gohan]

This PR contains a:

• [x] bugfix
• [ ] new feature
• [ ] code refactor
• [x] test update
• [ ] typo fix
• [ ] metadata update

bugfix for #39 https://github.com/webpack-contrib/node-loader/issues/39

Motivation / Use-Case

Fix: windows platform dlopen path concat

[codecov[bot]]

Codecov Report

Merging #40 (602c32b) into master (cd1e9c0) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #40   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           13        13           
  Branches         3         3           
=========================================
  Hits            13        13           

Impacted Files	Coverage Δ
src/index.js	100.00% <ø> (ø)

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[alexander-akait]

Thanks