Fix issue with destroying user record

Fixes #184

Fix issue where when calling `really_destroy!` on a user record will attempt
to save changes to a deleted record due to `scramble_email_and_password` callback.

This caused a runtime error "Can't modify frozen hash".

To fix this issue, return from the method immediate if the record has been destroyed.

Author: skukx

app/models/spree/user.rb

@@ -46,6 +46,8 @@ def set_login
     end
 
     def scramble_email_and_password
+      return true if destroyed?
+
       self.email = SecureRandom.uuid + "@example.net"
       self.login = email
       self.password = SecureRandom.hex(8)