Add NullCacheAdapter (#2636)

* Add NullCacheAdapter

* clear returns promise explicitly

* Add NullCacheAdapter accessor

spec/NullCacheAdapter.spec.js

@@ -0,0 +1,37 @@
+var NullCacheAdapter = require('../src/Adapters/Cache/NullCacheAdapter').default;
+
+describe('NullCacheAdapter', function() {
+  var KEY = 'hello';
+  var VALUE = 'world';
+
+  it('should expose promisifyed methods', (done) => {
+    var cache = new NullCacheAdapter({
+      ttl: NaN
+    });
+
+    // Verify all methods return promises.
+    Promise.all([
+      cache.put(KEY, VALUE),
+      cache.del(KEY),
+      cache.get(KEY),
+      cache.clear()
+    ]).then(() => {
+      done();
+    });
+  });
+
+  it('should get/set/clear', (done) => {
+    var cache = new NullCacheAdapter({
+      ttl: NaN
+    });
+
+    cache.put(KEY, VALUE)
+      .then(() => cache.get(KEY))
+      .then((value) => expect(value).toEqual(null))
+      .then(() => cache.clear())
+      .then(() => cache.get(KEY))
+      .then((value) => expect(value).toEqual(null))
+      .then(done);
+  });
+
+});