Use rfc3339 as default timestamp precision (#51)

According to https://docs.influxdata.com/influxdb/v1.7/tools/shell/#precision-rfc3339-h-m-s-ms-u-ns,
we should use rfc3339 instead of empty string as the default timestamp precision

Author: swr1bm86

Diff for: src/query/write_query.rs

@@ -92,7 +92,7 @@ impl WriteQuery {
 
     pub fn get_precision(&self) -> String {
         let modifier = match self.timestamp {
-            Timestamp::Now => return String::from(""),
+            Timestamp::Now => "rfc3339",
             Timestamp::Nanoseconds(_) => "ns",
             Timestamp::Microseconds(_) => "u",
             Timestamp::Milliseconds(_) => "ms",