Removing this feature causes boost in performance when using Ruby 1.9.

Ruby 1.9 started to do implicit conversions using `to_ary` and `to_str`
in some STDLIB methods (like Array#join). To do such implicit conversions,
Ruby 1.9 always dispatches the method and rescues the NoMethodError exception
in case one is raised.

Therefore, since the whiners feature defined NilClass#method_missing, such
implicit conversions for nil became much, much slower. In fact, just defining
NilClass#method_missing (even without the whiners feature) already causes a
massive slow down. Here is a snippet that shows such slow down:

    require "benchmark"
    Benchmark.realtime { 1_000.times { [nil,nil,nil].join } }

    class NilClass
      def method_missing(*args)
        raise NoMethodError
      end
    end

    Benchmark.realtime { 1_000.times { [nil,nil,nil].join } }