Fix redirections on Windows

[Xmader]

Thank you for fixing Backport Open Redirect vulnerability, but it breaks on Windows machines.

path.normalize("/path/to/file") returns \path\to\file on Windows, and it doesn't work in browsers.

[codecov-io]

Codecov Report

Merging #261 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #261   +/-   ##
=======================================
  Coverage   75.26%   75.26%           
=======================================
  Files          11       11           
  Lines         558      558           
  Branches      128      128           
=======================================
  Hits          420      420           
  Misses         51       51           
  Partials       87       87

Impacted Files	Coverage Δ
lib/ecstatic.js	72.36% <ø> (ø)	:arrow_up:

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[Xmader]

https://github.com/jfhbrook/node-ecstatic/issues/262

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 75.26%. Comparing base (ae7a39b) to head (4d75eef). Report is 4 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #261 +/- ## ======================================= Coverage 75.26% 75.26% ======================================= Files 11 11 Lines 558 558 Branches 128 128 ======================================= Hits 420 420 Misses 51 51 Partials 87 87 ```

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