only set ALPNProtocols when the property does not already exist

I guess maybe you could do `ALPNProtocols: null` to opt-out
of this new behavior. Not sure if it's really necessary but hey.

index.js

@@ -41,8 +41,10 @@ function HttpsProxyAgent(opts) {
   proxy.host = proxy.hostname || proxy.host;
   proxy.port = +proxy.port || (this.secureProxy ? 443 : 80);
 
-  if (this.secureProxy) {
-    proxy.ALPNProtocols = proxy.ALPNProtocols || ['http 1.1']
+  // ALPN is supported by Node.js >= v5.
+  // attempt to negotiate http/1.1 for proxy servers that support http/2
+  if (this.secureProxy && !('ALPNProtocols' in proxy)) {
+    proxy.ALPNProtocols = ['http 1.1']
   }
 
   if (proxy.host && proxy.path) {