Improve RSpec generated description (avoid should)

spec/support/fix_rspec_matchers_generated_description.rb

@@ -0,0 +1,8 @@
+module RSpec
+  module Matchers
+    def self.generated_description
+      return nil if last_expectation_handler.nil?
+      "must #{last_description}"
+    end
+  end
+end