Fix/issue666 (#889)

* cache key now can generate from query string for request with Get Methods and request content for requests with post methods

* MD5Helper Added. OutputCacheMiddleware now can generate cache key using method, url and content

* unit test created for CacheKeyGenerator

* CacheKeyGenerator Registered in OcelotBuilder as singletone

src/Ocelot.Cache.CacheManager/OcelotBuilderExtensions.cs

@@ -33,6 +33,10 @@
             builder.Services.RemoveAll(typeof(IOcelotCache<FileConfiguration>));
             builder.Services.AddSingleton<ICacheManager<FileConfiguration>>(fileConfigCacheManagerOutputCache);
             builder.Services.AddSingleton<IOcelotCache<FileConfiguration>>(fileConfigCacheManager);
+
+            builder.Services.RemoveAll(typeof(ICacheKeyGenerator));
+            builder.Services.AddSingleton<ICacheKeyGenerator, CacheKeyGenerator>();
+
             return builder;
         }
     }