Fix #18: --url localfile doesn't work on Windows.

[jschwartzentruber]

After this change yields:

windows:
D ffpuppet [2017-09-07 14:55:24] sending response with redirect url: 'file:///C:/Users/user/Desktop/tc.html'
linux:
D ffpuppet [2017-09-07 15:01:45] sending response with redirect url: 'file:/home/user/Desktop/tc.html'

Both redirects work properly.

[coveralls]

Coverage remained the same at 93.65% when pulling e583dc5 on truber-fix-windows-url-arg into a84915f on master.

[coveralls]

Coverage remained the same at 93.65% when pulling e583dc5 on truber-fix-windows-url-arg into a84915f on master.

[tysmith]

Thanks! I used the idea in this PR in commit ab8fc75.