py/stream: Properly handle .readline(maxsize) for text strings.
py/stream: Properly handle .readline(maxsize) for text strings.

For text strings, max size is given in characters, not bytes. As we actually
count bytes in the loop, we need to adjust counter by the size of current
utf-8 character in bytes.

Change-Id: I695a9fafbe0f9778dec7941f922ae18704754ff4
Signed-off-by: Paul Sokolovsky <9dc5061f178bb11813e14f1ed02ba27eaf21fa5d@users.sourceforge.net>

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython