Resolves https://jira.splunk.com/browse/DVPL-3641

splunklib.searchcommands | Writing messages header causes Getinfo probe failure

CAUSE:
+ outputheader=true must be set in commands.conf
+ messages header may only be sent during __EXECUTE__ phase, not __GETINFO__ phase.

TODO:
+ Issue feature ask for the search team: outputheader can be specified dynamically.

Signed-off-by: David Noble <dnoble@splunk.com>