[RedisCache] Exception throw when using ProtobufSerializer

[ianselmi]

When we try to use ProtobufSerializer with GetSet method the library throws the exception "The magic number in Gzip header is not correct"

this is an example

using IntelligentHack.IntelligentCache;
using ProtoBuf;
using StackExchange.Redis;
using System;

namespace RedisCacheSample
{
      class Program
      {
          static void Main(string[] args)
          {        
              var redisCache = new RedisCache(ConnectionMultiplexer.Connect("redis_cns"), "rd") { Serializer = new ProtobufSerializer()};

              redisCache.GetSet("Key1", () => new Entity { Prop1="test" }, TimeSpan.FromMinutes(1));
              var secondTime = redisCache.GetSet("Key1", () => new Entity { Prop1="test" }, TimeSpan.FromMinutes(1));
          }
      }
      [ProtoContract]
      public class Entity
      {
          [ProtoMember(1)]
          public string Prop1 { get; set; }
      }
}

the problem occurred when the entity is on redis and not the first time. PR #28 is a possible fix

[sklivvz]

fix incoming in #30