If the object path contains multiple slashes back-to-back, these used to
be wrongly merged by net.url.URL.String(). For example,

  account = "AUTH_aaa"
  container = "ccc"
  object = "foo///bar"

would become something like

  https://swift.example.com/v1/AUTH_aaa/ccc/foo/bar

With this change, we produce a more correct URL:

  https://swift.example.com/v1/AUTH_aaa/ccc/foo%2f%2f%2fbar