This patch doesn't change current behavior of "decode by chunk".
It's difficult to support changing encoding via header, like
Python's encoding cookie.

>>> import io
>>> buff = io.BytesIO("hello\nこんにちは".encode('utf-8'))
>>> f = io.TextIOWrapper(buff, encoding='ascii')
>>> f.readline()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/inada-n/work/python/cpython/Lib/encodings/ascii.py", line 26, in decode
    return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe3 in position 6: ordinal not in range(128)
>>>
>>> buff = io.BytesIO("hello\nこんにちは".encode('utf-8'))
>>> f = io.TextIOWrapper(buff, encoding='ascii')
>>> f.read(6)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/inada-n/work/python/cpython/Lib/encodings/ascii.py", line 26, in decode
    return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe3 in position 6: ordinal not in range(128)