Fix parsing json into optional types

Does not yet handle parsing missing values as `None` yet though.
Closes #25.

serde2/src/json/value.rs

@@ -78,6 +78,18 @@ impl de::Deserialize for Value {
                 Ok(Value::String(value))
             }
 
+            #[inline]
+            fn visit_none<E>(&mut self) -> Result<Value, E> {
+                Ok(Value::Null)
+            }
+
+            #[inline]
+            fn visit_some<D>(&mut self, deserializer: &mut D) -> Result<Value, D::Error>
+                where D: de::Deserializer,
+            {
+                de::Deserialize::deserialize(deserializer)
+            }
+
             #[inline]
             fn visit_seq<V>(&mut self, visitor: V) -> Result<Value, V::Error>
                 where V: de::SeqVisitor,