Prevent failures serializing commands with "holes" in arguments array.

This could be triggered when passing an array with "holes" to variadic commands.
Connection classes based on the protocol serialized exposed by phpiredis were
not affected by this bug.

Fixes #316.

tests/Predis/Protocol/Text/RequestSerializerTest.php

@@ -61,4 +61,27 @@ class RequestSerializerTest extends PredisTestCase
 
         $this->assertSame("*3\r\n$3\r\nSET\r\n$3\r\nkey\r\n$5\r\nvalue\r\n", $result);
     }
+
+    /**
+     * @group disconnected
+     */
+    public function testSerializerDoesNotBreakOnArgumentsWithHoles()
+    {
+        $serializer = new RequestSerializer();
+
+        $command = $this->getMock('Predis\Command\CommandInterface');
+
+        $command->expects($this->once())
+                ->method('getId')
+                ->will($this->returnValue('DEL'));
+
+        $command->expects($this->once())
+                ->method('getArguments')
+                ->will($this->returnValue(array(0 => 'key:1', 2 => 'key:2')));
+
+        $result = $serializer->serialize($command);
+
+        $this->assertSame("*3\r\n$3\r\nDEL\r\n$5\r\nkey:1\r\n$5\r\nkey:2\r\n", $result);
+    }
+
 }