Make signUp more robust

* Revert current user's password on failure if that's how it originally was
* Revert the current user's password on success

Author: grantland

Parse/src/main/java/com/parse/ParseUser.java

@@ -638,14 +638,19 @@ public Task<Void> then(Task<Void> task) throws Exception {
               synchronized (user.mutex) {
                 if (oldUsername != null) {
                   user.setUsername(oldUsername);
+                } else {
+                  user.revert(KEY_USERNAME);
                 }
                 if (oldPassword != null) {
                   user.setPassword(oldPassword);
+                } else {
+                  user.revert(KEY_PASSWORD);
                 }
                 user.restoreAnonymity(anonymousData);
               }
               return task;
             } else { // Success
+              user.revert(KEY_PASSWORD);
               revert(KEY_PASSWORD);
             }