Fixed allowDroppedNullPlaceholders in Readers.

(Based on 1.4.4)

The odd allowDroppedNullPlaceholders feature of Readers (corresponding to dropNullPlaceholders in Writers) can be helpful when parsing javascript as JSON, when null elements have been discarded.

This release fixes the Reader for several cases of the missing null. However, we do not guarantee that all corner cases are covered.

Also, re-add Json::Value::null for binary-compatibility.