Support x-forwarded-host header about ims-lti

[borrey]

Best workout around I found was from @FeynmanDNA https://github.com/NUS-ALSET/firebase-lti/blob/master/functions/lib/server.js

The important details are here extracted here:

//line 7
const _HmacSha1 = require('@dinoboff/ims-lti/lib/hmac-sha1');
//line 54
class HmacSha1 extends _HmacSha1 {
  protocol(req) {
    if (req.headers['x-appengine-https'] === 'on') {
      return 'https';
    }
    return super.protocol(req);
  }
}
//line 108
const provider = new lti.Provider(key, secret, {
      // Firebase functions is accessed via a reverse proxy. The lti signature
      // validation needs to use the original hostname and not the functions
      // server one.
      signer: new HmacSha1({trustProxy: true}),
      // Save nonces in datastore and ensure the request oauth1 nonce cannot be
      // used twice.
      nonceStore: database.nonceStore(key)
 });