Coerce string `index.columns` to Array before join (#378)

lib/annotate/annotate_models.rb

@@ -300,9 +300,9 @@ module AnnotateModels
       max_size = indexes.collect{|index| index.name.size}.max + 1
       indexes.sort_by(&:name).each do |index|
         if options[:format_markdown]
-          index_info << sprintf("# * `%s`%s:\n#     * **`%s`**\n", index.name, index.unique ? " (_unique_)" : "", index.columns.join("`**\n#     * **`"))
+          index_info << sprintf("# * `%s`%s:\n#     * **`%s`**\n", index.name, index.unique ? " (_unique_)" : "", Array(index.columns).join("`**\n#     * **`"))
         else
-          index_info << sprintf("#  %-#{max_size}.#{max_size}s %s %s", index.name, "(#{index.columns.join(",")})", index.unique ? "UNIQUE" : "").rstrip + "\n"
+          index_info << sprintf("#  %-#{max_size}.#{max_size}s %s %s", index.name, "(#{Array(index.columns).join(",")})", index.unique ? "UNIQUE" : "").rstrip + "\n"
         end
       end