7bit vs. UTF-8 forbidden characters detection

Hi all,

thanks for the very useful guessencoding tool. One idea for an improvement: There should be a method in CharsetToolkit that indicates that the test buffer contains only characters up to 0x7F.

For now you can't decide between these two cases when enforce8bit=true:

1. UTF-8 invalid bytes -> returns configured default charset
2. buffer contains no high bit, but after that there are possibly 8-bit or UTF-8 characters in File -> returns configured default charset, too

But in the first case I'd like to proceed with the default (8bit) charset, whereas in the second case I don't want to fall back to default, perhaps the buffer was just too short. Here I'd like to give UTF-8 a chance for the whole stream.

Switching to enforce8bit = false is no solution in case platform default charset is US-ASCII, because then
1. -> returns US-ASCII
2. -> returns US-ASCII too because enforce8bit=false

Proposed use case for a new method would be:

Charset used;
if(CharsetToolkit.seems7BitOnly(f, bufferLength))

else

Downside is that we cannot return guessed Charset and boolean validUTF8 status in one move and have to read f twice.

Thank you
Dominik