✅ Improve UIDPlusData#uid_mapping test coverage

Explicitly test how unsorted uid-sets are handled.

test/net/imap/test_uid_plus_data.rb

@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+require "net/imap"
+require "test/unit"
+
+class TestUIDPlusData < Test::Unit::TestCase
+
+  test "#uid_mapping with sorted source_uids" do
+    uidplus = Net::IMAP::UIDPlusData.new(
+      1, [19, 20, *(495..500)], [*(92..97), 100, 101],
+    )
+    assert_equal(
+      {
+         19 =>  92,
+         20 =>  93,
+        495 =>  94,
+        496 =>  95,
+        497 =>  96,
+        498 =>  97,
+        499 => 100,
+        500 => 101,
+      },
+      uidplus.uid_mapping
+    )
+  end
+
+  test "#uid_mapping for with source_uids in unsorted order" do
+    uidplus = Net::IMAP::UIDPlusData.new(
+      1, [*(495..500), 19, 20], [*(92..97), 100, 101],
+    )
+    assert_equal(
+      {
+        495 =>  92,
+        496 =>  93,
+        497 =>  94,
+        498 =>  95,
+        499 =>  96,
+        500 =>  97,
+         19 => 100,
+         20 => 101,
+      },
+      uidplus.uid_mapping
+    )
+  end
+
+end