Vulnerability Instance

Rack vulnerable to ReDoS in content type parsing (2nd degree polynomial)
### Summary

```ruby
module Rack
  class MediaType
    SPLIT_PATTERN = %r{\s*[;,]\s*}
```
The above regexp is subject to ReDos. 50K blank characters as a prefix to the header will take over 10s to split.

### PoC

A simple HTTP request with lots of blank characters in the content-type header:

```ruby
request["Content-Type"] = (" " * 50_000) + "a,"
```

### Impact

It's a very easy to craft ReDoS. Like all ReDoS the impact is debatable.