[rb] add enable_downloads to attr_accessor for all options classes

this only applies to chromium & firefox via grid, but it needs to be supported directly like this

Author: titusfortner

rb/lib/selenium/webdriver/common/options.rb

@@ -52,7 +52,7 @@ def safari(**opts)
         end
 
         def set_capabilities
-          (W3C_OPTIONS + self::CAPABILITIES.keys).each do |key|
+          (W3C_OPTIONS + GRID_OPTIONS + self::CAPABILITIES.keys).each do |key|
             next if method_defined? key
 
             define_method key do