ASCII escape control character prevents detection of encoding

[DRMacIver]

It appears to be the case that the presence of '\x1b' character (the escape key) in an ascii string prevents it from being recognised as ascii. e.g. for any n the following returns an encoding of None:

b'0' * n + b'\x1b'

This behaviour is present in master at ff1d917

[dan-blanchard]

Great find. That one probably has been around for a while!