Reduce buffer size to avoid pathological parsing

[CVE-2018-16470]

Revert "Merge pull request #1192 from jkowens/master"

This reverts commit c43217a.

lib/rack/multipart/parser.rb

@@ -5,7 +5,7 @@ module Multipart
  class MultipartPartLimitError < Errno::EMFILE; end
 
  class Parser
- BUFSIZE = 1_048_576
+ BUFSIZE = 16384
  TEXT_PLAIN = "text/plain"
  TEMPFILE_FACTORY = lambda { |filename, content_type|
  Tempfile.new(["RackMultipart", ::File.extname(filename.gsub("\0".freeze, '%00'.freeze))])