GELF appender: don't send full_message field

According to the GELF spec v1.1 [1], the full_message field in GELF is
optional. The log4js implemention until now has sent identitical
short_message and full_message fields. Since this does not add any new
information to the log message, I suggest that full_message be dropped
from GELF.

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[1] http://graylog2.org/gelf#specs
fix test/gelfAppender-test.js to reflect full_message being gone
Merge pull request #216 from 2ion/gelf-no-full-message

GELF appender: don't send full_message field