While WEBrick::HTTPRequest#body provides a Proc interface
for streaming large request bodies, clients must not force
the server to use an excessively large chunk size.

* lib/webrick/httprequest.rb (read_chunk_size): limit each
  read and block.call to :InputBufferSize in config.
* test/webrick/test_httpserver.rb (test_big_chunks): new test

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@62982 b2dd03c8-39d4-4d8f-98ff-823fe69b080e