Fix URI.merge/2 trailing slash handling at root path

[marcelotto]

Yet another fix of an issue in URI.merge/2 revealed by the W3C JSON-LD 1.1 test suite. The problem is that resolving a relative path with dot segments against a base URI incorrectly removes the trailing slash when the resolution ends at the root path.

Minimal example:

URI.merge("http://a/b", "..") |> to_string()
# Current: "http://a"    (incorrect)
# Expected: "http://a/"   (correct)

[josevalim]

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