Refactor AnnotateRoutes.header (ctran#714)

I refactored `AnnotateRoutes.header` for performance, and add `AnnotateRoutes.comment` to make code DRY.

lib/annotate/annotate_routes.rb

@@ -88,29 +88,37 @@ def header(options = {})
       end
       out << '' if magic_comments_map.any?
 
-      out += ["# #{options[:wrapper_open]}"] if options[:wrapper_open]
+      out << comment(options[:wrapper_open]) if options[:wrapper_open]
 
-      out += ["# #{options[:format_markdown] ? PREFIX_MD : PREFIX}" + (options[:timestamp] ? " (Updated #{Time.now.strftime('%Y-%m-%d %H:%M')})" : '')]
-      out += ['#']
+      out << comment(options[:format_markdown] ? PREFIX_MD : PREFIX) + (options[:timestamp] ? " (Updated #{Time.now.strftime('%Y-%m-%d %H:%M')})" : '')
+      out << comment
       return out if routes_map.size.zero?
 
       maxs = [HEADER_ROW.map(&:size)] + routes_map[1..-1].map { |line| line.split.map(&:size) }
 
       if options[:format_markdown]
         max = maxs.map(&:max).compact.max
 
-        out += ["# #{content(HEADER_ROW, maxs, options)}"]
-        out += ["# #{content(['-' * max, '-' * max, '-' * max, '-' * max], maxs, options)}"]
+        out << comment(content(HEADER_ROW, maxs, options))
+        out << comment(content(['-' * max, '-' * max, '-' * max, '-' * max], maxs, options))
       else
-        out += ["# #{content(routes_map[0], maxs, options)}"]
+        out << comment(content(routes_map[0], maxs, options))
       end
 
-      out += routes_map[1..-1].map { |line| "# #{content(options[:format_markdown] ? line.split(' ') : line, maxs, options)}" }
-      out += ["# #{options[:wrapper_close]}"] if options[:wrapper_close]
+      out += routes_map[1..-1].map { |line| comment(content(options[:format_markdown] ? line.split(' ') : line, maxs, options)) }
+      out << comment(options[:wrapper_close]) if options[:wrapper_close]
 
       out
     end
 
+    def comment(row = '')
+      if row == ''
+        '#'
+      else
+        "# #{row}"
+      end
+    end
+
     # TODO: write the method doc using ruby rdoc formats
     # This method returns an array of 'real_content' and 'header_position'.
     # 'header_position' will either be :before, :after, or