title: OptionParser throwing 'Missing Argument' for no reasons
body: <p>I only have 1 possible option and it is parsed as following: </p>
  
  <pre class="lang-rb prettyprint-override"><code>  def parse_options
      options = {}
      options[:markdown] = false
      OptionParser.new do |opts|
        opts.on('-md', '--markdown', 'Use Markdown Syntax') do
          options[:markdown] = true
        end
      end.parse!
    end
  </code></pre>
  
  <p>As you can see the option doesn't require any argument. What I find even stranger is this works: </p>
  
  <pre><code>command -md
  </code></pre>
  
  <p>but this throws the exception:</p>
  
  <pre><code>command --markdown
  
  in `parse_options': missing argument: --markdown (OptionParser::MissingArgument)
  </code></pre>
  
  <p>Any ideas? I read the doc and multiple examples but can't figure it out. </p>