A hyphen and a equal should not be converted to heading

Hello.
I guess a hyphen and a equal should not be converted to heading.
For example, this code will convert to heading.
```ruby
redcarpet = Redcarpet::Markdown.new(Redcarpet::Render::HTML)
redcarpet.render("-") #=> "<h2></h2>\n"
# But, I'd expected "<p>-</p>\n"
```
Please review, thanks.