When the root certificate has an EC key and he intermediate has an
RSA key, the signature algorithm of the leafs should be the default
one, SHA256WithRSA, instead of the one that the intermediate has.
Fixes#1033
- cs.NegotiatedProtocolIsMutual has been deprecated but we still build
in travis with 1.14 so for now we'll ignore this linting error
- string(int) was resolving to string of a single rune rather than
string of digits -> use fmt.Sprint
