Fixes https://github.com/Mahoney/slf4j-test/issues/4 - Detect throwable as last varargs element if the format string has n-1 parameters.
With thanks to https://github.com/philipa
Fixed an issue where null arguments to a logging event and null values in the MDC caused a NullPointerException due to Guava’s strict attitude to nulls. Null arguments now appear as Optional.absent() and null values in the MDC as the String “null”.
