Fix InfluxDB line protocol encoder

In particular, it didn't properly escape tag values with double quotes.

Change-Id: I8328f8cf23e49a3aa63a86e1c6866df36f8fe08d
diff --git a/tests/lua/test_afd.lua b/tests/lua/test_afd.lua
index 478a36c..1016984 100644
--- a/tests/lua/test_afd.lua
+++ b/tests/lua/test_afd.lua
@@ -54,7 +54,7 @@
         assertEquals(last_injected_msg.Type, 'afd_metric')
         assertEquals(last_injected_msg.Fields.value, consts.OKAY)
         assertEquals(last_injected_msg.Fields.hostname, 'node-1')
-        assertEquals(last_injected_msg.Fields.notification_handler, 'mail')
+        assertEquals(last_injected_msg.Fields.notification_handler, nil)
         assertEquals(last_injected_msg.Payload, '{"alarms":[]}')
     end