fixed array to string conversion bug for the log writer

http://forum.kohanaframework.org/discussion/11567/kohana-3-3-strtr-array-to-string-conversion-issue-in-kohana_log_writer/p1

https://bugs.php.net/bug.php?id=63893

http://php.net/manual/en/migration54.incompatible.php

On Servers with php 5.4 running, the error log function throws an Exception with the notice that some parts of the message (trace from the exception) can`t be castet to string.

With this Update from the First Forum link the bug will be fixed