async_wget2 and async_wget2_data additionalHeaders argument

Update async_wget2 and async_wget2_data to allow passing additional headers to http requests.
Add support for range response status 206 to async_wget2_data.

The aim is primarily to support http ranges with 'GET' requests.
the try catch statement is here to prevent JSON.parse(str) from throwing exception occuring in cases where additionalHeader arg is not a valid Json string, for instance when it is empty string