Checking availability before calling Log::flushSharedContext() method

[ajaxray]

Type: minor bug fix
Location: Illuminate\Queue\QueueServiceProvider::registerWorker()

There are some edge cases where $app['log'] may not contain flushSharedContext() method.

In the next statement, in the case of withoutContext() (line 204 of QueueServiceProvider), it's checking for method existence. Similarly, I think i'd be nice to check method existence before calling this function too.

Background

I was trying to use Queue in a Laravel Zero app without installing log component. And was getting this error:

Call to undefined method Psr\Log\NullLogger::flushSharedContext()
at vendor/illuminate/queue/QueueServiceProvider.php:201

Then I checked and found that, the Psr\Log\NullLogger actually don't have this method.