Fix URI.merge/2 handling of base with trailing dots

[marcelotto]

This PR fixes a bug in URI.merge/2 that was revealed via the W3C test suite of the JSON-LD 1.1 algorithms. The merging with a base ending in dot segments without a trailing slash was not handled correctly.

Example:

URI.merge("http://a/bb/ccc/.", "g") |> to_string()
# Current: "http://a/bb/ccc/+/g"  (incorrect)
# Expected: "http://a/bb/ccc/g"   (correct)

[josevalim]

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