Change multipart handling to check for object#read instead of specific object types

This change allows passing params[:file] in a Rails 3.2 upload directly into Nestful's post function (Rails 3.2 wraps the uploaded file in yet another object type).  Really all we need to know is whether the object responds to read, in other words; does it act like a StringIO object?

The changeset has a bunch of deleted whitespace because SublimeText strips extra whitespace out of blank lines automatically.