Value: call _store_fix only if value was changed by user fix code
Value: limit the number of times a fix can be tried
fix test: fix code was wrong and did not really fix the test value
Value: avoid undef warning
Value::_store_value: propagate notify_change only if necessary
Value: croak if notify_change is called for nothing
Backend::Yaml: load_data now requires named parameters
test: fix test of store return value
Value: store always returns 1
multistrap test: fixed