Fix capitalize for single codepoint (#13268)

Author: josevalim

lib/elixir/lib/string.ex

@@ -966,6 +966,7 @@ defmodule String do
 
   def capitalize(string, mode) when is_binary(string) do
     case :unicode_util.gc(string) do
+      [gc] -> grapheme_to_binary(:string.titlecase([gc]))
       [gc, rest] -> grapheme_to_binary(:string.titlecase([gc])) <> downcase(rest, mode)
       [gc | rest] -> grapheme_to_binary(:string.titlecase([gc])) <> downcase(rest, mode)
       [] -> ""

lib/elixir/test/elixir/string_test.exs

@@ -254,6 +254,7 @@ defmodule StringTest do
 
   test "capitalize/1" do
     assert String.capitalize("") == ""
+    assert String.capitalize("1") == "1"
     assert String.capitalize("abc") == "Abc"
     assert String.capitalize("ABC") == "Abc"
     assert String.capitalize("c b a") == "C b a"