binary.toByteString() only handles string values

[grob]

The current implementation of binary.toByteString() always converts the argument to a String (it was a String.prototype method before b085a7a - see #409). This is properly documented, but nevertheless the simple change from the prototype method toByteString() to binary.toByteString() broke stick's etag middleware in case of binary responses.

I'm wondering if we should change binary.toByteString() to add a check whether the first argument is an instance of Binary (and thus call Binary.prototype.toByteString()), because i sense there will be some more regressions coming up. Same with binary.toByteArray().