Fix handling of backslashes on Windows (#47)
Fix handling of backslashes on Windows (#47)

Replace backslashes in the path with forward slashes so the result is a
valid URL. This is using the same approach for processCopy() as was used
in processRebase().
In conjunction with the previous commit, this gets npm test passing all
33 tests on Windows (unless you have a space character in your upstream
path!).

postcss/postcss-url