Make WEBrick handler use Tempfile for large input

WEBrick handler is reading the entire request body into a giant string in memory. Usually this is avoided by setting an upper limit for input held in memory and switching to a Tempfile once that limit is exceeded. Puma and Thin both have this maximum value set to 1024 * (80 + 32) #=> 114688 bytes, so I cargo-culted that number as well.