Miscalculating in _read_until

My _read_until:

```
def _read_until(self, delimiter, callback):
    self._read_buffer += self.recv(4096)
    pos = self._read_buffer.find(delimiter)
    if pos >= 0:
        pos += len(delimiter)
    if pos > 0:
        data = self._read_buffer[:pos]
        self._read_buffer = self._read_buffer[pos:]
        if data:
            callback(data)
```
