fix(storage): improve typings for UploadTask.on()

Author: jsayol

typings/app.d.ts

@@ -398,7 +398,7 @@ declare namespace firebase.storage {
   interface UploadTask {
     cancel ( ) : boolean ;
     catch (onRejected : (a : Error ) => any ) : firebase.Promise < any > ;
-    on (event : firebase.storage.TaskEvent , nextOrObserver ? : firebase.Observer<firebase.storage.UploadTaskSnapshot, Error> | null , error ? : ( (a : Error ) => any ) | null , complete ? : ( ( ) => any ) | null ) : Function ;
+    on (event : firebase.storage.TaskEvent , nextOrObserver ? : firebase.Observer<firebase.storage.UploadTaskSnapshot, Error> | ((snapshot: firebase.storage.UploadTaskSnapshot) => any) , error ? : ( (a : Error ) => any ) | null , complete ? : ( ( ) => any ) | null ) : Function ;
     pause ( ) : boolean ;
     resume ( ) : boolean ;
     snapshot : firebase.storage.UploadTaskSnapshot ;

typings/firebase.d.ts

@@ -398,7 +398,7 @@ declare namespace firebase.storage {
   interface UploadTask {
     cancel ( ) : boolean ;
     catch (onRejected : (a : Error ) => any ) : firebase.Promise < any > ;
-    on (event : firebase.storage.TaskEvent , nextOrObserver ? : firebase.Observer<firebase.storage.UploadTaskSnapshot, Error> | null , error ? : ( (a : Error ) => any ) | null , complete ? : ( ( ) => any ) | null ) : Function ;
+    on (event : firebase.storage.TaskEvent , nextOrObserver ? : firebase.Observer<firebase.storage.UploadTaskSnapshot, Error> | ((snapshot: firebase.storage.UploadTaskSnapshot) => any) , error ? : ( (a : Error ) => any ) | null , complete ? : ( ( ) => any ) | null ) : Function ;
     pause ( ) : boolean ;
     resume ( ) : boolean ;
     snapshot : firebase.storage.UploadTaskSnapshot ;