Fix Markdown formatting of narrow table columns (#1430)

lib/livebook/live_markdown/markdown_helpers.ex

@@ -353,6 +353,7 @@ defmodule Livebook.LiveMarkdown.MarkdownHelpers do
       cells
       |> Enum.map(&IO.iodata_length/1)
       |> Enum.max()
+      |> max(3)
     end)
   end
 

test/livebook/live_markdown/markdown_helpers_test.exs

@@ -212,6 +212,16 @@ defmodule Livebook.LiveMarkdown.MarkdownHelpersTest do
       assert markdown == reformat(markdown)
     end
 
+    test "table with narrow column" do
+      markdown = """
+      | x   | y   | x and y |
+      | :-: | --- | ------- |
+      | 1   | 1   | 0       |\
+      """
+
+      assert markdown == reformat(markdown)
+    end
+
     test "basic unordered list" do
       markdown = """
       * Olafur Arnalds