In practice, we always act as an HTTP/1.1 client, for compatibility
with servers which ignore protocol version. Sending the version in
the request will avoid problems with servers which don't ignore it.
HTTP/1.0 can still be forced using a stream context option.
Closes GH-5899.
On Windows the built-in webserver doesn't prevent directory traversal when
backslashes are used as path component separators. Even though that is not a
security issue (the CLI webserver is meant for testing only), we fix that by
replacing backslashes in the path with slashes on Windows, because backslashes
may be valid characters for file names on other systems, but not on Windows.
