[firebase_ui] create account: email does not get updated when changed after name and password have been entered.

[Soundbytes]

This will surely not occur often but you should always expect people to do strange things when using your app.

Steps to reproduce:

• start the example app
• hit 'Sign in with email' button
• enter emal
• enter name
• enter password
• then before submitting change email entry
• submit

Expected behavior:
app creates a new firebase user with the updated credentials.

What's happening instead:
App creates a new user with from the entries before the change was made.