Don't treat asterisks in list items as italic in Markdown

[rspeicher]

Fixes an issue where asterisks used for list items were converted to underscores.

Before:

  1) Slack::Messages::Formatting#markdown doesn't treat list items as text formatting
     Failure/Error: expect(formatting.markdown(msg)).to eq msg

       expected: "Une liste:\n\n* Article 1\n* Article 2\n* Article 3\n"
            got: "Une liste:\n\n_ Article 1\n_ Article 2\n* Article 3\n"

       (compared using ==)

       Diff:
       @@ -1,6 +1,6 @@
        Une liste:

       -* Article 1
       -* Article 2
       +_ Article 1
       +_ Article 2
        * Article 3

     # ./spec/slack/messages/formatting_spec.rb:161:in `block (3 levels) in <top (required)>'

Introduced in https://github.com/slack-ruby/slack-ruby-client/pull/520.

[coveralls]

Pull Request Test Coverage Report for Build 11729294352

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on rs-asterisk-lists at 89.336%

---

Totals
Change from base Build 11707538954:	89.3%
Covered Lines:	5395
Relevant Lines:	6039

---

💛 - Coveralls

[dblock]

Perfect, thank you.