Fix WSGI headers parse and readline implementation

[us3r64]

Socket data might be already read into self._buffer, especially with https://github.com/adafruit/Adafruit_CircuitPython_WSGI/blob/main/adafruit_wsgi/esp32spi_wsgiserver.py#L67 implementation.

Reading everything from socket is going to include HTTP body also but discarded if self._buffer is not rechecked.

Fix: #185