Fix crypto hash update invalid arg type error

[mems]

Error happen when you provide a number as custom key:

TypeError [ERR_INVALID_ARG_TYPE]: The "data" argument must be one of type string, TypedArray, or DataView
    at Hash.update (internal/crypto/hash.js:53:11)
    at ...

To fix that, just force key parts to be a String

[coveralls]

Coverage remained the same at 97.543% when pulling 0716956 on mems:patch-1 into bc48c35 on AdamPflug:master.