Handle NULL byte in multipart file name

[casperisfine]

Rack 1.x was handling those just fine.

Rack 2.x blow up with:

    ArgumentError: string contains null byte
    ~/.gem/ruby/2.2.3/gems/rack-2.0.1/lib/rack/multipart/parser.rb:11 in `extname`
    ~/.gem/ruby/2.2.3/gems/rack-2.0.1/lib/rack/multipart/parser.rb:11 in `in `block in <class`
    ~/.gem/ruby/2.2.3/gems/rack-2.0.1/lib/rack/multipart/parser.rb:137 in `call`
    ~/.gem/ruby/2.2.3/gems/rack-2.0.1/lib/rack/multipart/parser.rb:137 in `on_mime_head`
    ~/.gem/ruby/2.2.3/gems/rack-2.0.1/lib/rack/multipart/parser.rb:262 in `handle_mime_head`
    ~/.gem/ruby/2.2.3/gems/rack-2.0.1/lib/rack/multipart/parser.rb:215 in `block in run_parser`
    ~/.gem/ruby/2.2.3/gems/rack-2.0.1/lib/rack/multipart/parser.rb:208 in `loop`
    ~/.gem/ruby/2.2.3/gems/rack-2.0.1/lib/rack/multipart/parser.rb:208 in `run_parser`
    ~/.gem/ruby/2.2.3/gems/rack-2.0.1/lib/rack/multipart/parser.rb:191 in `on_read`
    ~/.gem/ruby/2.2.3/gems/rack-2.0.1/lib/rack/multipart/parser.rb:70 in `parse`
    ~/.gem/ruby/2.2.3/gems/rack-2.0.1/lib/rack/multipart.rb:52 in `extract_multipart`
    ~/.gem/ruby/2.2.3/gems/rack-2.0.1/lib/rack/request.rb:472 in `parse_multipart`
    ~/.gem/ruby/2.2.3/gems/rack-2.0.1/lib/rack/request.rb:335 in `POST`

What is happening is that File.extname internally convert the ruby string to a C string, and C strings can't contain NULL bytes.

cc @rafaelfranca @kirs

[rafaelfranca]

LGTM