Use adapter lint for rack adapter
Fix IO response bodies for Rack/WEBrick/Mongrel

There were three issues that needed to be resolved.

1. Precedence issue in `IOEncoder#each`
2. IOEncoder needs to define `empty?` if it has a content length
   chunked transfer encoding is not set on IO response bodies the same
   way that it is for enum/proc/fiber, so `ensure_content_length` ends
   up calling `set_content_length`, which calls `empty?`
3. Adapters shouldn't use ChunkedBody unless Transfer-Encoding says so

The fixes I put in are just workarouns. I think that some real thought
should be put into better long term fixes.