[rb] don't redefine capabilities if not necessary

Author: titusfortner

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

@@ -25,6 +25,8 @@ class Options
 
       def self.set_capabilities
         (W3C_OPTIONS + self::CAPABILITIES.keys).each do |key|
+          next if method_defined? key
+
           define_method key do
             @options[key]
           end