you can't delete a field's value

[mrjones-plip]

If you have entered a value in a field and saved it to the database, you can not save a null value back to that field.

steps to reproduce

1. create a row in a databse
2. go back and edit it and try to put no value (delete all text) and click save

expected - when you view the row, the field is empty
actual - the old value is still in the field

fix - in cleanseValuesAgainstSchema() we're checking if a field is defined like this:

if (isset($values[$key])) {

If $values[$key] == null then this isset will be false. we actually just want to see if it's defined, not if it's null. so let's switch that line to this:

if (array_key_exists($key, $values)) {

and we'll be all bettah!