mb_check_encoding should not be part of Safe

[bolmstedt]

mb_check_encoding returns true if the tested string is the correct encoding and false if it's not, which is not an error. I do have to say that the description of the return values on php.net is unclear, but the function's purpose is pretty clear from function description, naming and boolean return value.

What's more, Safe's implementation has void as return type.

I paraphrase what I said on #80: "It is not an error [..], therefore it seems to be outside of Safe's scope."

[moufmouf]

👍 Completely ok. I'll remove it in next release.

[moufmouf]

Closed in #90.
Thanks!