Prefer point timestamp in InfluxDB accumulator

This commit implements of using point timestamp, if it exists,
for InfluxDB records. Earlier, we used timestamp of Heka message.

Change-Id: Ib830ada5659799ab7ff3bb80fd5ec00942805290
diff --git a/heka/files/lua/filters/influxdb_accumulator.lua b/heka/files/lua/filters/influxdb_accumulator.lua
index 470bdef..9bdeec8 100644
--- a/heka/files/lua/filters/influxdb_accumulator.lua
+++ b/heka/files/lua/filters/influxdb_accumulator.lua
@@ -111,9 +111,10 @@
                 point.tags[k] = v
             end
         end
+
         accumulator:append(
             encoder:encode_datapoint(
-                msg_timestamp,
+                point["timestamp"] or msg_timestamp,
                 point.name,
                 point.value or point.values,
                 point.tags))