Support root arrays in JSONInput

If a root array '[13, 14, 15]' is given to the JSONInput it will fail to
parse it because it is expecting an object. Root arrays are conformable to
the old JSON specification that the version of RapidJSON cereal uses
implements

include/cereal/archives/json.hpp

@@ -414,7 +414,10 @@ namespace cereal
         itsReadStream(stream)
       {
         itsDocument.ParseStream<0>(itsReadStream);
-        itsIteratorStack.emplace_back(itsDocument.MemberBegin(), itsDocument.MemberEnd());
+        if (itsDocument.IsArray())
+          itsIteratorStack.emplace_back(itsDocument.Begin(), itsDocument.End());
+        else
+          itsIteratorStack.emplace_back(itsDocument.MemberBegin(), itsDocument.MemberEnd());
       }
 
       //! Loads some binary data, encoded as a base64 string
@@ -685,7 +688,10 @@ namespace cereal
       //! Loads the size for a SizeTag
       void loadSize(size_type & size)
       {
-        size = (itsIteratorStack.rbegin() + 1)->value().Size();
+        if (itsIteratorStack.size() == 1)
+          size = itsDocument.Size();
+        else
+          size = (itsIteratorStack.rbegin() + 1)->value().Size();
       }
 
       //! @}