concurrent/cached_value: Avoid computing twice

Also, clear `@computation` after we obtained the value, as we no longer
need it.

Refs #300

lib/i18n/tasks/concurrent/cached_value.rb

@@ -20,8 +20,12 @@ def initialize(&computation)
     def get
       return @result unless @result == NULL
       @mutex.synchronize do
-        @result = @computation.call
+        if @result == NULL
+          @result = @computation.call
+          @computation = nil
+        end
       end
+      @result
     end
   end
 end