in measure, allow `Option<d128>` fields with "D" prefix

example:

```
let a: Option<d128> = Some(d128::from_str("123.456").unwrap());
let b: Option<d128> = None;
measure!(influx, meas, D(a), D(b));
```

in the `None` case, no field is added at all.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "influx-writer"
-version = "0.10.0"
+version = "0.10.1"
 authors = ["Jonathan Strong <jonathan.strong@gmail.com>"]
 edition = "2018"
 

src/lib.rs

@@ -140,6 +140,13 @@ macro_rules! measure {
     (@ea u, $meas:ident, $k:expr, $v:expr) => { $meas = $meas.add_field($k, $crate::OwnedValue::Uuid($v)) };
     (@ea b, $meas:ident, $k:expr, $v:expr) => { $meas = $meas.add_field($k, $crate::OwnedValue::Boolean(bool::from($v))) };
 
+    (@ea D, $meas:ident, $k:expr, $v:expr) => { 
+        match $v {
+            Some(v) => { $meas = $meas.add_field($k, $crate::OwnedValue::D128(v)) }
+            None => {}
+        }
+    };
+
     (@as_expr $e:expr) => {$e};
 
     (@count_tags) => {0usize};