GOCBC-789: Send using POST instead of POTS in FreezePlan

Change-Id: Iec4894fe043d2242a1f65aade3561bb6a24fb0e9
Reviewed-on: http://review.couchbase.org/122167
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Charles Dixon <chvckd@gmail.com>

cluster_searchindexes.go

@@ -604,7 +604,7 @@ func (sm *SearchIndexManager) FreezePlan(indexName string, opts *AllowQueryingSe
 
 	return sm.performControlRequest(
 		span.Context(),
-		"POTS",
+		"POST",
 		fmt.Sprintf("/api/index/%s/planFreezeControl/freeze", indexName),
 		opts.Timeout,
 		opts.RetryStrategy)