Mention the correct default contentEncoding (aes128gcm) in the README (#576)

Also, avoid base64-decoding a key which is then re-encoded later.

Author: 4r7d3c0

README.md

@@ -184,7 +184,7 @@ request only. This overrides any API key set via `setGCMAPIKey()`.
 - **TTL** is a value in seconds that describes how long a push message is
 retained by the push service (by default, four weeks).
 - **headers** is an object with all the extra headers you want to add to the request.
-- **contentEncoding** is the type of push encoding to use (e.g. 'aesgcm', by default, or 'aes128gcm').
+- **contentEncoding** is the type of push encoding to use (e.g. 'aes128gcm', by default, or 'aesgcm').
 - **proxy** is the [HttpsProxyAgent's constructor argument](https://github.com/TooTallNate/node-https-proxy-agent#new-httpsproxyagentobject-options)
 that may either be a string URI of the proxy server (eg. http://< hostname >:< port >)
 or an "options" object with more specific properties.

src/vapid-helper.js

@@ -187,7 +187,6 @@ function getVapidHeaders(audience, subject, publicKey, privateKey, contentEncodi
   validatePublicKey(publicKey);
   validatePrivateKey(privateKey);
 
-  publicKey = urlBase64.decode(publicKey);
   privateKey = urlBase64.decode(privateKey);
 
   if (expiration) {
@@ -215,13 +214,13 @@ function getVapidHeaders(audience, subject, publicKey, privateKey, contentEncodi
 
   if (contentEncoding === WebPushConstants.supportedContentEncodings.AES_128_GCM) {
     return {
-      Authorization: 'vapid t=' + jwt + ', k=' + urlBase64.encode(publicKey)
+      Authorization: 'vapid t=' + jwt + ', k=' + publicKey
     };
   }
   if (contentEncoding === WebPushConstants.supportedContentEncodings.AES_GCM) {
     return {
       Authorization: 'WebPush ' + jwt,
-      'Crypto-Key': 'p256ecdsa=' + urlBase64.encode(publicKey)
+      'Crypto-Key': 'p256ecdsa=' + publicKey
     };
   }