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 to get "<p>-</p>\n"
```
Also, this behavior leads to incorrect output of html_toc.
```ruby
toc = Redcarpet::Markdown.new(Redcarpet::Render::HTML_TOC)
toc.render(<<-markdown) #=> "<ul>\n<li>\n<a href=\"#hello-world\">Hello World</a>\n<ul>\n<li>\n<a href=\"#\"></a>\n</li>\n</ul>\n</li>\n</ul>\n"
# Hello World

-
markdown
```
Please review, thanks.