Add Logger::setTimezone()

Add a setter method to allow changing the timezone used when creating
new log records. This provides a means to correct a behavioral change
introduced by 6cbdc04 where use of the PHP runtime default timezone
provided by `date_default_timezone_get()` was introduced. In most
environments the new default behavior is preferable, but if an
application allows the default timezone to vary between requests it
causes confusing log output.

src/Monolog/Logger.php

@@ -614,4 +614,14 @@ class Logger implements LoggerInterface
     {
         return $this->addRecord(static::EMERGENCY, $message, $context);
     }
+
+    /**
+     * Set the timezone to be used for the timestamp of log records.
+     *
+     * @param string $tz Timezone name
+     */
+    public static function setTimezone($tz)
+    {
+        self::$timezone = new \DateTimeZone($tz);
+    }
 }