fix(http/file_server): use platform specific resolve

[Chocobo1]

On Windows, running posix.resolve("C:\\Users\\abc") within C:\Users\abc\Desktop\folderabc would resolve to
C:\Users\abc\Desktop\folderabc/C:\Users\abc which is not expected.

[CLAassistant]

All committers have signed the CLA.

[kt3k]

Can you write a test case for this? ignore: Deno.build.os !== "windows" option in test definition would probably be useful

[Chocobo1]

Can you write a test case for this?

Sorry, I'm not sure what kind of test would do. Any ideas?

[kt3k]

Start the server like this line:

deno_std/http/file_server_test.ts

Line 488 in 2d9dd29

const fileServer = new Deno.Command(Deno.execPath(), {

With the first argument as a path starting with C:/. And then check a correct file is served, and stop the server.

[Chocobo1]

I've added a test, please take a look.

[kt3k]

Thanks for adding a test case! LGTM