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

test/Ocelot.UnitTests/Cache/CacheKeyGeneratorTests.cs

@@ -0,0 +1,34 @@
+using System.Net.Http;
+using Microsoft.AspNetCore.Http;
+using Ocelot.Cache;
+using Ocelot.Middleware;
+using Shouldly;
+using TestStack.BDDfy;
+using Xunit;
+
+namespace Ocelot.UnitTests.Cache {
+    public class CacheKeyGeneratorTests {
+        private readonly ICacheKeyGenerator _cacheKeyGenerator;
+        private readonly DownstreamContext _downstreamContext;
+
+        public CacheKeyGeneratorTests() {
+            _cacheKeyGenerator = new CacheKeyGenerator();
+            _cacheKeyGenerator = new CacheKeyGenerator();
+            _downstreamContext = new DownstreamContext(new DefaultHttpContext()) {
+                DownstreamRequest = new Ocelot.Request.Middleware.DownstreamRequest(new HttpRequestMessage(HttpMethod.Get, "https://some.url/blah?abcd=123"))
+            };
+        }
+
+        [Fact]
+        public void should_generate_cache_key_from_context() {
+            this.Given(x => x.GivenCacheKeyFromContext(_downstreamContext))
+                .BDDfy();
+        }
+
+        private void GivenCacheKeyFromContext(DownstreamContext context) {
+            string generatedCacheKey = _cacheKeyGenerator.GenerateRequestCacheKey(context);
+            string cachekey = MD5Helper.GenerateMd5("GET-https://some.url/blah?abcd=123");
+            generatedCacheKey.ShouldBe(cachekey);
+        }
+    }
+}