Add Default for Box<RawValue>

Equivalent behavior to Default for Value.
diff --git a/src/raw.rs b/src/raw.rs
index f63d102..989c39d 100644
--- a/src/raw.rs
+++ b/src/raw.rs
@@ -149,6 +149,12 @@
     }
 }
 
+impl Default for Box<RawValue> {
+    fn default() -> Self {
+        RawValue::from_borrowed("null").to_owned()
+    }
+}
+
 impl Debug for RawValue {
     fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
         formatter