Fix memcached bucket replace with cas.

Motivation
----------
The MemcachedBucket.Replace(key, value, cas) untested and doing an Add
instead of a Replace. This patch fixes it and adds the appropriate tests.

Modifications
-------------
MemcachedBucket.Replace(key, value, cas) was corrected and tests were added to
MemcachedBucket.Tests.

Results
-------
MemcachedBucket.Replace(key, value, cas) is fixed and has tests to
assert that it works correctly.

Change-Id: I3a948b701605f9ff03a536da5287c72b7f1ba6b5
Reviewed-on: http://review.couchbase.org/48487
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>

Src/Couchbase.Tests/Core/Buckets/MemcachedBucketTests.cs

@@ -121,6 +121,45 @@ public void When_Key_Exists_Replace_Succeeds()
             }
         }
 
+        [Test]
+        public void When_Cas_Has_Changed_Replace_Fails()
+        {
+            const string key = "CouchbaseBucket.When_Cas_Has_Changed_Replace_Fails";
+            using (var bucket = _cluster.OpenBucket("memcached"))
+            {
+                bucket.Remove(key);
+                var set = bucket.Insert(key, "value");
+                Assert.IsTrue(set.Success);
+
+                var upsert = bucket.Upsert(key, "newvalue");
+                Assert.IsTrue(upsert.Success);
+
+                var replace = bucket.Replace(key, "should fail", set.Cas);
+                Assert.IsFalse(replace.Success);
+            }
+        }
+
+        [Test]
+        public void When_Cas_Has_Not_Changed_Replace_Succeeds()
+        {
+            const string key = "CouchbaseBucket.When_Cas_Has_Not_Changed_Replace_Succeeds";
+            using (var bucket = _cluster.OpenBucket("memcached"))
+            {
+                bucket.Remove(key);
+                var set = bucket.Insert(key, "value");
+                Assert.IsTrue(set.Success);
+
+                var get = bucket.Get<string>(key);
+                Assert.AreEqual(get.Cas, set.Cas);
+
+                var replace = bucket.Replace(key, "should succeed", get.Cas);
+                Assert.True(replace.Success);
+
+                get = bucket.Get<string>(key);
+                Assert.AreEqual("should succeed", get.Value);
+            }
+        }
+
         [Test]
         public void When_Key_Exists_Delete_Returns_Success()
         {

Src/Couchbase/MemcachedBucket.cs

@@ -369,7 +369,7 @@ public IDocumentResult<T> Replace<T>(IDocument<T> document)
         /// <returns>An object implementing the <see cref="IOperationResult{T}"/>interface.</returns>
         public IOperationResult<T> Replace<T>(string key, T value, ulong cas)
         {
-            var operation = new Add<T>(key, value, null, _converter, _transcoder, _operationLifespanTimeout);
+            var operation = new Replace<T>(key, value, cas, null, _converter, _transcoder, _operationLifespanTimeout);
             return _requestExecuter.SendWithRetry(operation);
         }