Use RUBY_ENGINE instead of RUBY_PLATFORM to detect JRuby.
lib/nuggets/ruby_mixin.rb: Extracted from lib/nuggets/ruby.rb.
lib/nuggets/ruby_mixin.rb: Added RUBY_PLATFORM, accounting for JRuby.
lib/nuggets/io/null_mixin.rb: Use RubyMixin::RUBY_PLATFORM.