Support custom primary key in accounts table on MySQL

If one wants to use UUID primary keys on MySQL, one possibility is using
a string column and generating UUIDs in Ruby.

  # table definition
  create_table :accounts do
    String :id, primary_key: true
    # ...
  end

  # rodauth configuration
  before_create_account do
    account[:id] = SecureRandom.uuid
  end

However, Sequel's `#insert` appears to return `0` when the primary key
is something other than an autoincrementing integer on MySQL. This
causes Rodauth's `#save_account` to overwrite the generated UUID with
`0` on the account hash, causing problems down the line in things like
account verification.

We fix this by having `#save_account` only set the `:id` if it hasn't
already been set.

lib/rodauth/features/create_account.rb

@@ -106,7 +106,7 @@ def save_account
       end
 
       if id
-        account[account_id_column] = id
+        account[account_id_column] ||= id
       end
 
       id && !raised