Make the channel for log mail driver configurable

Allows users to specify the channel the log the emails when using the `log` driver.

src/Illuminate/Mail/TransportManager.php

@@ -160,7 +160,11 @@ protected function createSparkPostDriver()
      */
     protected function createLogDriver()
     {
-        return new LogTransport($this->app->make(LoggerInterface::class));
+        $channel = $this->app['config']['mail.log_channel'];
+
+        return new LogTransport(
+            $this->app->make(LoggerInterface::class)->channel($channel)
+        );
     }
 
     /**

tests/Mail/MailLogTransportTest.php

@@ -0,0 +1,50 @@
+<?php
+
+namespace Illuminate\Tests\Mail;
+
+use Monolog\Logger;
+use Psr\Log\LoggerInterface;
+use Orchestra\Testbench\TestCase;
+use Monolog\Handler\StreamHandler;
+use Monolog\Handler\RotatingFileHandler;
+use Illuminate\Mail\Transport\LogTransport;
+
+class MailLogTransportTest extends TestCase
+{
+    public function testGetLogTransportWithDefaultChannel()
+    {
+        $manager = $this->app['swift.transport'];
+
+        $transport = $manager->driver('log');
+        $this->assertInstanceOf(LogTransport::class, $transport);
+
+        $logger = $this->readAttribute($transport, 'logger');
+        $this->assertInstanceOf(LoggerInterface::class, $logger);
+
+        $this->assertInstanceOf(Logger::class, $monolog = $logger->getLogger());
+        $this->assertCount(1, $handlers = $monolog->getHandlers());
+        $this->assertInstanceOf(RotatingFileHandler::class, $handlers[0]);
+    }
+
+    public function testGetLogTransportWithConfiguredChannel()
+    {
+        $this->app['config']->set('mail.log_channel', 'mail');
+        $this->app['config']->set('logging.channels.mail', [
+            'driver' => 'single',
+            'path' => 'mail.log',
+        ]);
+
+        $manager = $this->app['swift.transport'];
+
+        $transport = $manager->driver('log');
+        $this->assertInstanceOf(LogTransport::class, $transport);
+
+        $logger = $this->readAttribute($transport, 'logger');
+        $this->assertInstanceOf(LoggerInterface::class, $logger);
+
+        $this->assertInstanceOf(Logger::class, $monolog = $logger->getLogger());
+        $this->assertCount(1, $handlers = $monolog->getHandlers());
+        $this->assertInstanceOf(StreamHandler::class, $handler = $handlers[0]);
+        $this->assertEquals('mail.log', $handler->getUrl());
+    }
+}