Allow using string for :urls if it consists of only one element

I think this will not work if `@urls` be a hash. `overwrite_file_path` will never call.
```ruby
    def initialize(app, options={})
      @app = app
      @urls = options[:urls] || options[:url] || ["/favicon.ico"]
      @urls = Array(@urls) unless @urls.kind_of?(Hash)
      @index = options[:index]
      root = options[:root] || Dir.pwd
      ...