fix

In value.h, ValueConstIterator can convert to ValueIterator, I think that is a bug. the correct way is ValueIterator can convert to ValueConstIterator.
diff --git a/include/json/value.h b/include/json/value.h
index 237aa53..7f3ad9b 100644
--- a/include/json/value.h
+++ b/include/json/value.h
@@ -738,6 +738,7 @@
   typedef ValueConstIterator SelfType;
 
   ValueConstIterator();
+  ValueConstIterator(ValueIterator const& other);
 
 private:
 /*! \internal Use by Value to create an iterator.
@@ -787,7 +788,7 @@
   typedef ValueIterator SelfType;
 
   ValueIterator();
-  ValueIterator(const ValueConstIterator& other);
+  explicit ValueIterator(const ValueConstIterator& other);
   ValueIterator(const ValueIterator& other);
 
 private: