Use move ctor in append()

Author: cdunn2001

src/lib_json/json_value.cpp

@@ -1148,7 +1148,7 @@ Value const& Value::operator[](CppTL::ConstString const& key) const
 Value& Value::append(const Value& value) { return (*this)[size()] = value; }
 
 #if JSON_HAS_RVALUE_REFERENCES
-  Value& Value::append(Value&& value) { return (*this)[size()] = value; }
+  Value& Value::append(Value&& value) { return (*this)[size()] = std::move(value); }
 #endif
 
 Value Value::get(char const* key, char const* cend, Value const& defaultValue) const