Changing chunk size to 32M for Swift chunked download.

When calling `fetch_object()` with a chunk size, pyrax will create a new request for each chunk as opposed to streaming the output from a raw request. In order to minimize the overhead of (potentially) many new HTTP requests (authentication, extra information, etc.), we're increasing the chunk size to 32M for Swift downloads.

In the future, this requirement may be removed depending on the implementation in pyrax / request.