Force string to ASCII-8BIT before passing to UUID initializer

Hey -- not sure what changed (maybe the newer version of thrift-client?) but when plugging the latest head of cassandra-cql into my app, it's blowing up when trying to cast UUIDs as the strings are UTF-8 encoded. So this is just a small patch to force the string to UTF-8 before passing them to the UUID initializer, which seems to fix the problem.