Fix string length

[visavi]

In different encodings such as UTF-8, the validator incorrectly believed the string length

[koenpunt]

Thanks for your contribution. Please add a test for the faulty behavior.

[visavi]

before and after

[xak2000]

This change assumes the encoding is always UTF-8. But it is not.
In my project, the DB connection's encoding is KOI8-R for example (I can't change it to UTF-8 for many reasons).
And it will fail to check string length after this change.

A agree that UTF-8 is more common these days, but I think this problem requires more work. At least configurable encoding that respects DB connection's encoding.