Add "default" as accepted string for "connections".

Using "default" returns a connection factory instance with the default
configuration. Basically it is no different than omitting "connections"
in the client options array but it can be useful when applications want
to automatically configure Predis to use phpiredis when it is loaded:

$client = new Predis\Client('tcp://127.0.0.1', [
  'connections' =>
      extension_loaded('phpiredis')
        ? 'phpiredis'
        : 'default'
]);

This is in response to ISSUE #397.

src/Configuration/Option/Connections.php

@@ -112,6 +112,9 @@ class Connections implements OptionInterface
                 $factory->define('unix', 'Predis\Connection\PhpiredisSocketConnection');
                 break;
 
+            case 'default':
+                return $factory;
+
             default:
                 throw new \InvalidArgumentException(sprintf(
                     '%s does not recognize `%s` as a supported configuration string', static::class, $value

tests/Predis/Configuration/Option/ConnectionsTest.php

@@ -88,6 +88,33 @@ class ConnectionsTest extends PredisTestCase
         $this->assertSame($default, $factory);
     }
 
+    /**
+     * @group disconnected
+     */
+    public function testAcceptsStringDefaultToReturnConnectionFactoryWithDefaultConfiguration()
+    {
+        $options = $this->getMockBuilder('Predis\Configuration\OptionsInterface')->getMock();
+
+        $default = $this->getMockBuilder('Predis\Connection\FactoryInterface')->getMock();
+        $default
+            ->expects($this->never())
+            ->method('define');
+
+        $option = $this->getMockBuilder('Predis\Configuration\Option\Connections')
+        ->setMethods(array('getDefault'))
+        ->getMock();
+        $option
+            ->expects($this->once())
+            ->method('getDefault')
+            ->with($options)
+            ->will($this->returnValue($default));
+
+        $factory = $option->filter($options, 'default');
+
+        $this->assertInstanceOf('Predis\Connection\FactoryInterface', $factory);
+        $this->assertSame($default, $factory);
+    }
+
     /**
      * @group disconnected
      */