Blake2b argument checking

[bpvgoncalves]

Hello.
I've been looking at this package for the last couple of weeks because I wanted to use argon2 as a kdf for a project of mine and used your version as the base for it.
In the meantime I've found an issue with the 'key' argument type checking on blake2b R function: instead of checking the 'key' for types string and raw, the 'x' (input to be hashed) is checked twice.

I have this fixed on my fork (branch fix-blake2b) and I'm more than happy to open a PR if you so wish.
Also happy to contribute back the developments I did over your code if you find them useful to include in the package.

Best Regards.
Bruno