Slighly more robust tofu detection

The checking of if platform supports trust-on-first-use is broken on node 6.
We need a more robust way of detecting this, but until then this will have to do.

Author: pontusmelke

src/v1/internal/features.js

@@ -27,7 +27,7 @@ const FEATURES = {
       // that supports reading the whole certificate, eg this commit:
       // https://github.com/nodejs/node/commit/345c40b6
       let desc = require('tls').TLSSocket.prototype.getPeerCertificate.toString();
-      return desc.startsWith("function getPeerCertificate(detailed)");
+      return desc.indexOf("function getPeerCertificate(detailed)") >= 0;
     } catch( e ) {
       return false;
     }