Bind to beforeunload instead of unload to fix firefox issues

Some firefox users reported odd connection issues caused by an
unload listener
ref: phoenixframework/phoenix_live_view#1425

Author: chrismccord

assets/js/phoenix.js

@@ -868,7 +868,7 @@ export class Socket {
       this.decode = this.defaultDecoder
     }
     if(phxWindow && phxWindow.addEventListener){
-      phxWindow.addEventListener("unload", e => {
+      phxWindow.addEventListener("beforeunload", e => {
         if(this.conn){
           this.unloaded = true
           this.abnormalClose("unloaded")
@@ -1625,4 +1625,4 @@ class Timer {
       this.callback()
     }, this.timerCalc(this.tries + 1))
   }
-}
+}