Fix connected property value (#453)

## What does this PR do?

The `kuzzle.connected` property which delegate to the protocol was testing a wrong `protocol.state` value (`'online'` instead of `'connected'`).

Author: Aschen

src/protocols/abstract/common.js

@@ -40,7 +40,7 @@ class AbstractWrapper extends KuzzleEventEmitter {
   }
 
   get connected () {
-    return this.state === 'online';
+    return this.state === 'connected';
   }
 
   get pendingRequests () {

test/protocol/common.test.js

@@ -18,6 +18,18 @@ describe('Common Protocol', () => {
     sendSpy = sinon.spy(protocol, 'send');
   });
 
+  describe('#connected', () => {
+    it('should return true if the protocol state is "connected"', () => {
+      protocol.state = 'connected';
+
+      should(protocol.connected).be.True();
+
+      protocol.state = 'disconnected';
+
+      should(protocol.connected).be.False();
+    });
+  });
+
   describe('#query', () => {
 
     beforeEach(() => {