AsciiDoc renderer renders [upperalpha] ordered lists with lowercase characters

[wuan]

When creating an ordered list with uppercase characters using

[upperalpha]
. one
. two
. three

the GitHub renderer uses lowercase characters:

a. one b. two c. three

See also that example Gist: https://gist.github.com/wuan/392df0b754d28bfdb782349e97abcf7a and the official AsciiDoc documentation describing that feature: https://docs.asciidoctor.org/asciidoc/latest/lists/ordered/#styles.