[racket] multipart http requests

Consider implementing it from scratch, in an application-specific way.  
Implementing the writing is pretty simple and straightforward (easier 
than the reading).

Just be careful that your boundary string does not occur in any part 
(which is a weakness of the protocol, IMHO): you can either sanity-check 
all your encoded parts, or you can cross your fingers and make a big 
random string.

Neil V.