UTF8 characters at 8192 byte boundary will cause errors

```
'utf-8' codec can't decode byte 0xc3 in position 8191: unexpected end of data: UnicodeDecodeError
Traceback (most recent call last):
File "/var/task/serverless_sdk/__init__.py", line 54, in wrapped_handler
return user_handler(event, context)
File "/var/task/handler.py", line 162, in slack
offers = get_lunch_offers(city)
File "/var/task/handler.py", line 100, in get_lunch_offers
offers = streaming_download_and_parse_offers(url)
File "/var/task/handler.py", line 89, in streaming_download_and_parse_offers
res_data = response.read(8192).decode('utf-8')
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xc3 in position 8191: unexpected end of data
```

My assumption is that there's a UTF8 character consisting of multiple bytes at the boundary of the byte array. The fix would be to read 8192 bytes, try decode, if fails, read 8192 mode and retry.