fixed separator on windows for result.json

[gregorymaertens]

To have each bundle into separate subfolders, it's currently possible to have such a thing having the bundle named like 'js/vendor'.
On windows there is an error where \ is used in the result.json file. E.g.
config.js:

    bundle : {
        'js/vendor' : {
            scripts : [
...

produces

  "js/vendor": {
    "scripts": "js\vendor-9de064e6.js"
  }

this PR covers this point. The unit test doesn't cover exactly the above case but uses the pathPrefix option for windows like separator.

[chmontgomery]

Thanks for the PR! I'll review shortly

[chmontgomery]

published in v2.19.1