fix: urls were backslashed on windows

While Microsoft Windows uses the backslash `\` as a directory separator, urls use the forwardslash. This pull request simply transforms generated urls before injection, to valid urls.

before on windows:
```
<script src="\fixtures\test.js"></script>
```

after:
```
<script src="/fixtures/test.js"></script>
```

**note**: the unit test does byte to byte comparison and so is vulnerable to differences in line feeds and carriage returns across platforms. 
```
test.equal(actual, expected, 'should insert the available files in the file.');
```

I suggest doing this in your command line, for a more robust comparison:
```
git diff test\fixtures\file.html test\expected\file.html
```