fix(Cluster): empty key name was sent to random nodes

Before this fix, empty key names (""/null/undefined) was
sent to random nodes. That should not be the case when
key name is an empty string.

Related with #390.

Thank Aditya-Chowdhry for addressing this issue!

Author: luin

lib/command.js

@@ -84,10 +84,10 @@ Command.prototype.initPromise = function () {
 Command.prototype.getSlot = function () {
   if (typeof this._slot === 'undefined') {
     var key = this.getKeys()[0];
-    if (key) {
-      this.slot = calculateSlot(key);
-    } else {
+    if (key == null) {
       this.slot = null;
+    } else {
+      this.slot = calculateSlot(key);
     }
   }
   return this.slot;

test/unit/command.js

@@ -77,12 +77,18 @@ describe('Command', function () {
   describe('#getSlot()', function () {
     it('should return correctly', function () {
       expectSlot('123', 5970);
+      expectSlot(123, 5970);
       expectSlot('ab{c', 4619);
       expectSlot('ab{c}2', 7365);
       expectSlot('ab{{c}2', 2150);
       expectSlot('ab{qq}{c}2', 5598);
       expectSlot('ab}', 11817);
       expectSlot('encoding', 3060);
+      expectSlot(true, 13635);
+      expectSlot('true', 13635);
+      expectSlot('', 0);
+      expectSlot(null, 0);
+      expectSlot(undefined, 0);
 
       function expectSlot(key, slot) {
         expect(new Command('get', [key]).getSlot()).to.eql(slot);