Don't trigger beforeSave on login

[simonbengtsson]

The Parse iOS SDK (and maybe others?) sends a "create user request" even though the user already exists. Basically using a create request to login. It is handled in parse-server right now by short circuiting the request:

// RestWrite.js::321
if (!this.query) {
  // We're signing up, but this user already exists. Short-circuit
  delete results[0].password;
  this.response = {
    response: results[0],
    location: this.location()
  };
  return;
}

The problem is that this check happens after the beforeSave is triggered which means it will be triggered for every login. This PR fixes that and short circuits before the beforeSave is called.

One side effect of this PR is that the beforeSave is not triggered when authData, username or password is invalid. I thought this made sense, but it might be up for discussion.

This PR brings parse-server closer to parse.com as beforeSave was not called on login there.

[nlutsenko]

Looks it needs some rebasing, but I agree with restoring the proper intention of not using triggering beforeSave on user login.
Can we also add few tests here?

[simonbengtsson]

With the latest updates I am not seeing this anymore and beforeSave is not called on login.

[nlutsenko]

Awesome!