Log4j 1.2 bridge should ignore case in properties file keys

Log4j 1.2 bridge should ignore case in properties file keys.

This will allow both of these styles to work:

log4j.appender.FILE_APPENDER.file=${java.io.tmpdir}/${hadoop.log.file}

log4j.appender.FILE_APPENDER.File=${java.io.tmpdir}/${hadoop.log.file}

Notice the "file" vs. "File"