LOG4NET-405 fixed default encoding to be UTF8

According to http://msdn.microsoft.com/en-us/library/system.text.encoding.default UTF8 is a better choice than Default.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4net/trunk@1541408 13f79535-47bb-0310-9956-ffa450edef68