Expose tiny-http's chunked_threshold value to the RequestData struct

[jsrobson10]

Fixes

• Audio metadata preload fails: no Content-Length header sent for matched asset #249

Changes

These changes allow the chunked_threshold value in tiny-http to be overridden per request.

This was achieved by adding an extra field to RequestData. I would have liked to add it to Request instead, but RequestData has private fields which means the changes could be made without causing downstream breakage.

Examples

Force the Content-Length header to always be used:

ResponseBody::from_reader_and_size(reader, buff.len())
    .with_chunked_threshold(usize::max_value())

[jsrobson10]

I've just merged the upstream changes into this PR and verified that all tests still pass.