Test attachment upload via multipart/form-data

This is used in Futon attachment upload.
Test form-urlencoded doc update

Update handlers have access to a form object parsed from an
application/x-www-form-urlencoded request.
Merge branch 'various-tests' of https://github.com/jo/couchdb

* 'various-tests' of https://github.com/jo/couchdb:
  Test form-urlencoded doc update
  Test attachment upload via multipart/form-data

Closes PR #55