Allow json missing fields to be treated as a null

Closes #34.

src/json/de.rs

@@ -543,6 +543,7 @@ impl<'a, Iter> de::MapVisitor for MapVisitor<'a, Iter>
         }
 
         if self.de.eof() {
+            println!("here3");
             return Err(self.de.error(ErrorCode::EOFWhileParsingValue));
         }
 
@@ -573,6 +574,31 @@ impl<'a, Iter> de::MapVisitor for MapVisitor<'a, Iter>
             Err(self.de.error(ErrorCode::TrailingCharacters))
         }
     }
+
+    fn missing_field<V>(&mut self, _field: &'static str) -> Result<V, Error>
+        where V: de::Deserialize,
+    {
+        // See if the type can deserialize from a unit.
+        struct UnitDeserializer;
+
+        impl de::Deserializer for UnitDeserializer {
+            type Error = Error;
+
+            fn visit<V>(&mut self, mut visitor: V) -> Result<V::Value, Error>
+                where V: de::Visitor,
+            {
+                visitor.visit_unit()
+            }
+
+            fn visit_option<V>(&mut self, mut visitor: V) -> Result<V::Value, Error>
+                where V: de::Visitor,
+            {
+                visitor.visit_none()
+            }
+        }
+
+        Ok(try!(de::Deserialize::deserialize(&mut UnitDeserializer)))
+    }
 }
 
 struct EnumVisitor<'a, Iter: 'a> {