HMAC fix for the sign and verify

[olegbespalov]

What?

This change fixes a degradation that happened after merging the #61

I found it while working on the ECDSA support, and it seems like ECDSA was also a reason why the test case was disabled #41.

Why?

Without the change our example erroring with:

./k6 run examples/sign_verify/sign-verify-hmac.js

INFO[0000] {"name":"InvalidAccessError","message":"key algorithm does not describe a HMAC key"}  source=console

[mstoykov]

LGTM!

btw usually single method interfaces have er at the end of the single method name as their name. That is to say that hasHash usually should be hasher.

Not really all that important, but maybe we can fix it in another PR

[olegbespalov]

Yep, and I'm trying to follow this go-convention. However, here it's not the hasher but something that has a hash() string method