I chose char* as `PEM_read_bio_PrivateKey` doesn't accept a const ([here](https://github.com/joyent/node/blob/master/deps/openssl/openssl/crypto/pem/pem_pkey.c#L75))

I suppose another option would be to cast with `const_cast`, my assumption is that this would be safe as it isn't stored as a const in the first place?