Fix URI.merge/2 handling of authorityless base

[marcelotto]

Yet another fix of an issue in URI.merge/2. The problem concerns merging against a base with non-hierarchical URI schemes without authority (like "urn:"). According to the RFC 3986 merging algorithm, when dealing with base URIs that have a scheme but no authority, the path should be directly resolved. The current behavior however prepends a leading slash, which is neither standards-compliant nor useful in practice.

Example:

URI.merge("ex:", "test") |> to_string()
# Current: "ex:/test"  (incorrect)
# Expected: "ex:test"  (correct)

This should be the last fix in the series of corrections to the URI.merge/2 algorithm, as this makes the last failing test related to URI merging of the W3C JSON-LD 1.1 test suite pass.

Additionally, this PR reorganizes the grown URI.merge/2 tests into a describe block for better structure and readability.

[josevalim]

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