Downgrade INFO string to ASCII

On rare occasions we have seen Redis v2.4.17 instances with invalid data in the INFO response: `used_memory_peak_human:\x90\f\x83\xFB\x83\u007F`

The 'used_memory_peak_human' key should be a string on the form "12345.67K". For our instance the `used_memory_peak:18446744073706301432` is very big, and looking at redis.c (for both 2.4.17 and 2.6.7) I suspect that `bytesToHuman()` is buggy and doesn't deal with such big values.

This patch simply downgrades the info string to ASCII when the encoding is invalid.