Inspired by Tim Peters' 1.14->1.15 checkin to asynchat

Inspired by Tim Peters' 1.14->1.15 checkin to asynchat
Quoting orignal message:
'''
Fix from the Madusa mailing list:
    http://groups.yahoo.com/group/medusa/message/333

It's clear that Medusa should not be checking for an empty buffer
via "buf is ''".  The patch merely changes "is" to "==".  However,
there's a mystery here all the same:  Python attempts to store null
strings uniquely, so it's unclear why "buf is ''" ever returned
false when buf actually was empty.  *Some* string operations produce
non-unique null strings, e.g.
'''