Keep Document value unchanged on parse error, fixes #437

Keeping the DOM unchanged in case of an error is the intended
behaviour according to the [documentation] [1].

Instead of forcing the value to `kNullType` before starting the
parsing, store the parsed value upon success via regular move.

[1]: https://miloyip.github.io/rapidjson/md_doc_dom.html#ParseError

include/rapidjson/document.h

@@ -1886,14 +1886,13 @@ public:
     */
     template <unsigned parseFlags, typename SourceEncoding, typename InputStream>
     GenericDocument& ParseStream(InputStream& is) {
-        ValueType::SetNull(); // Remove existing root if exist
         GenericReader<SourceEncoding, Encoding, StackAllocator> reader(
             stack_.HasAllocator() ? &stack_.GetAllocator() : 0);
         ClearStackOnExit scope(*this);
         parseResult_ = reader.template Parse<parseFlags>(is, *this);
         if (parseResult_) {
             RAPIDJSON_ASSERT(stack_.GetSize() == sizeof(ValueType)); // Got one and only one root object
-            this->RawAssign(*stack_.template Pop<ValueType>(1));    // Add this-> to prevent issue 13.
+            ValueType::operator=(*stack_.template Pop<ValueType>(1));// Move value from stack to document
         }
         return *this;
     }