Auto re-subscribe upon re-connect (#340)

* Auto re-subscribe upon re-connect

* Update FIXME

* newHead -> subscribeNewHead in example

docs/examples/promise/02_listen_to_blocks/index.js

@@ -10,7 +10,7 @@ async function main () {
   // Subscribe to the new headers on-chain. The callback is fired when new headers
   // are found, the call itself returns a promise with a subscription that can be
   // used to unsubscribe from the newHead subscription
-  const subscriptionId = await api.rpc.chain.newHead((header) => {
+  const subscriptionId = await api.rpc.chain.subscribeNewHead((header) => {
     console.log(`best #${header.blockNumber}`);
   });