Monkey patch to fix WEBrick chunking semantics.

 * Previously proposed in #707, unfortunately that patch caused double encoding
 * Fixes #707, #618
 * The longevity of this patch is dubious. If WEBrick makes identical semantics
   modifications as I think should be done, this patch will have no effect. If
   WEBrick introduces changes to internal header handling, class structure, etc,
   we'll break.
Merge pull request #710 from rack/webrick-chunking

Monkey patch to fix WEBrick chunking semantics.