Windows doesn't allow `\ / : * ? " < > |` in filenames so create
the fixture files at runtime and ignore the incompatible ones when
running on Windows.
RFC 3986[1] allows sub-delim characters in path segments unencoded,
however Rack::File requires them to be encoded so we use URI's
unescape method to leave them alone and then escape them again.
Also since the path gets passed to Dir[] we need to escape any glob
characters in the path.
[1]: http://www.ietf.org/rfc/rfc3986.txt