Unauthorized error to_s breaks if request is nil

[rhymes]

According to

oauth-ruby/lib/oauth/errors/unauthorized.rb

Lines 2 to 11 in 200b109

	class Unauthorized < OAuth::Error
	attr_reader :request
	def initialize(request = nil)
	@request = request
	end
	def to_s
	[request.code, request.message] * " "
	end
	end

the argument request defaults to nil, but if called with nil, .to_s will break with

     NoMethodError:
       undefined method `code' for nil:NilClass