Incorrect line character when merging a single and double line box

[goblinfactory]

            var console = new MockConsole(12, 10);
            var line = new Draw(console, LineThickNess.Single, Merge);
            line.Box(0, 0, 8, 6, LineThickNess.Double);
            line.Box(3, 3, 11, 9, LineThickNess.Single);
           
         // produces the following output with invalid vertical line
       
            console.Buffer.Should().BeEquivalentTo(new[]
            {
               "╔═══════╗   ",
               "║       ║   ",
               "║       ║   ",
               "║  ┌────│──┐",
               "║  │    ║  │",
               "║  │    ║  │",
               "╚══╪════╝  │",
               "   │       │",
               "   │       │",
               "   └───────┘"
            });

Note the invalid vertical line at the intersection.

[goblinfactory]

will be fixed when completing #65