v2.3.20

Refactored HttpRangeReader/HttpReader:

• use a GET request instead of a HEAD request to get the size of a zip file from the Content-Range response header when retrieving it with HttpRangeReader or HttpReader and the option useRangeHeader or forceRangeRequests set to true. However, a HEAD request will still be sent if the response to the GET request does not include the size of the content in the Content-Range header or if this header is not present (thanks @bennbollay, see #291 for more info).
• add the support of the headers option when using HttpRangeReader or HttpReader with the option useXHR set to true (works like the fetch API).