Fix URI.merge/2 for hostless absolute base URIs (#14344)

Author: marcelotto

lib/elixir/lib/uri.ex

@@ -903,7 +903,7 @@ defmodule URI do
   @spec merge(t | binary, t | binary) :: t
   def merge(uri, rel)
 
-  def merge(%URI{host: nil}, _rel) do
+  def merge(%URI{scheme: nil}, _rel) do
     raise ArgumentError, "you must merge onto an absolute URI"
   end
 

lib/elixir/test/elixir/uri_test.exs

@@ -460,6 +460,11 @@ defmodule URITest do
              "https://images.example.com/t/1600x/https://images.example.com/foo.jpg"
   end
 
+  test "merge/2 with host-less URIs" do
+    assert URI.merge("tag:example", "foo") |> to_string == "tag:foo"
+    assert URI.merge("tag:example", "#fragment") |> to_string == "tag:example#fragment"
+  end
+
   test "merge/2 (with RFC examples)" do
     # These are examples from:
     #