Fix extra slash being added on root path URIs

The following bug was introduced in d6de7e83:

{https, undefined, "www.example.com", 443, "/", undefined, undefined},
  was being built as
"https://www.example.com//"

This corrects the issue by not appending a slash when the root
path ("/") is specified (it should behave the same as passing an
empty ("") path).

Preserve the trailing slash on paths when building URIs

Paths with and without trailing slashes are distinct, therefore a
trailing slash in the path should be preserved when building the URI.