Content-Length prediction

2.2.0 (with typing and empty file size bugs fixed in 2.2.2 and 2.2.1)

• Zip file size can now be predicted by the predictLength function before creating it.
• Added an options parameter to downloadZip.
  • options.length can be set to include a "Content-Length" header in the Response.
  • options.metadata can instead be given the same argument as predictLength, as a shortcut to compute and set the Content-Length.