IsValid returns true for invalid binary data

[erezarnon]

> ObjectId.isValid("�½��ļ���.zip")
true
> ObjectId("�½��ļ���.zip")
ObjectID(undefinedbdundefinedundefinedundefinedundefinedundefinedundefined2e7a6970)

[niftylettuce]

PR welcome to fix this!

[niftylettuce]

The PR #46 broke all tests. I am going to revert it in the interim.

I also noticed we need to merge this 418sec#2.